diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 79fffa96b..ca3d20951 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,6 +1,6 @@
-
 Validator:
 * enforce minValue and maxValue for decimal and Quantity types
+* add checks for whitespace in base64 content (warning in <R5)
 
 Other code changes:
 * Fix trailing slashes for JAVA_HOME tests
diff --git a/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/VersionUtilities.java b/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/VersionUtilities.java
index 18640920f..b9edecf17 100644
--- a/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/VersionUtilities.java
+++ b/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/VersionUtilities.java
@@ -479,5 +479,16 @@ public class VersionUtilities {
     return mm1 != null && mm2 != null && mm1.equals(mm2);
   }
 
+  public static boolean isR5VerOrLater(String version) {
+    if (version == null) {
+      return false;
+    }
+    if (version.startsWith(CURRENT_VERSION) || version.equals("current")) {
+      return true;
+    }
+    String v = getMajMin(version);
+    return v.compareTo("4.5") >= 0; 
+  }
+
 
 }
\ No newline at end of file
diff --git a/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java b/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java
index f489c39eb..5eaf4c981 100644
--- a/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java
+++ b/org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/i18n/I18nConstants.java
@@ -688,7 +688,8 @@ public class I18nConstants {
   public static final String TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_MIN_NO_CONVERT = "TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_MIN_NO_CONVERT";
   public static final String TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_VALUE_WRONG = "TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_VALUE_WRONG";
   public static final String TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_VALUE_WRONG_UCUM = "TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_VALUE_WRONG_UCUM";
-
+  public static final String TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_ERROR = "TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_ERROR";
+  public static final String TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_WARNING = "TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_WARNING";
 }
 
 
diff --git a/org.hl7.fhir.utilities/src/main/resources/Messages.properties b/org.hl7.fhir.utilities/src/main/resources/Messages.properties
index 83d301ed5..e9e2d0e97 100644
--- a/org.hl7.fhir.utilities/src/main/resources/Messages.properties
+++ b/org.hl7.fhir.utilities/src/main/resources/Messages.properties
@@ -698,4 +698,6 @@ TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_NO_UCUM_SVC = There is no UCUM service, and the
 TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_MIN_NO_CONVERT = Unable to convert value {0} from unit {1} to maxValue unit {2} based on UCUM definitions; maximum value is not valid
 TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_VALUE_WRONG = The value in the instance ({2}) is greater than the specified maximum value ({3})
 TYPE_SPECIFIC_CHECKS_DT_QTY_MAX_VALUE_WRONG_UCUM = The value in the instance ({0} {1}) is greater than the specified maxValue ({2} {3}) after UCUM conversion 
+TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_ERROR = Base64 values are not allowed to contain any whitespace (per RFC 4648). Note that non-validating readers are encouraged to accept whitespace anyway
+TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_WARNING = Base64 values SHOULD not contain any whitespace (per RFC 4648). Note that non-validating readers are encouraged to accept whitespace anyway
 
diff --git a/org.hl7.fhir.validation/src/main/java/org/hl7/fhir/validation/instance/InstanceValidator.java b/org.hl7.fhir.validation/src/main/java/org/hl7/fhir/validation/instance/InstanceValidator.java
index 439a471bb..485cee2c6 100644
--- a/org.hl7.fhir.validation/src/main/java/org/hl7/fhir/validation/instance/InstanceValidator.java
+++ b/org.hl7.fhir.validation/src/main/java/org/hl7/fhir/validation/instance/InstanceValidator.java
@@ -2216,6 +2216,13 @@ public class InstanceValidator extends BaseValidator implements IResourceValidat
         if (!ok) {
           String value = encoded.length() < 100 ? encoded : "(snip)";
           rule(errors, IssueType.INVALID, e.line(), e.col(), path, false, I18nConstants.TYPE_SPECIFIC_CHECKS_DT_BASE64_VALID, value);
+        } else {
+          boolean wsok = base64HasWhitespace(encoded);
+          if (VersionUtilities.isR5VerOrLater(this.context.getVersion())) {
+            rule(errors, IssueType.INVALID, e.line(), e.col(), path, false, I18nConstants.TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_ERROR);            
+          } else {
+            warning(errors, IssueType.INVALID, e.line(), e.col(), path, false, I18nConstants.TYPE_SPECIFIC_CHECKS_DT_BASE64_NO_WS_WARNING);            
+          }
         }
         if (ok && context.hasExtension("http://hl7.org/fhir/StructureDefinition/maxSize")) {
           int size = countBase64DecodedBytes(encoded);
@@ -2420,6 +2427,20 @@ public class InstanceValidator extends BaseValidator implements IResourceValidat
     return ok;
   }
 
+  private boolean base64HasWhitespace(String theEncoded) {
+    if (theEncoded == null) {
+      return false;
+    }
+    for (int i = 0; i < theEncoded.length(); i++) {
+      char nextChar = theEncoded.charAt(i);
+      if (Character.isWhitespace(nextChar)) {
+        return true;
+      }
+    }
+    return false;
+
+  }
+
 
   private int countBase64DecodedBytes(String theEncoded) {
     Base64InputStream inputStream = new Base64InputStream(new ByteArrayInputStream(theEncoded.getBytes(StandardCharsets.UTF_8)));