diff --git a/wp-includes/blocks.php b/wp-includes/blocks.php
index 0bbc6a345c..4795631de7 100644
--- a/wp-includes/blocks.php
+++ b/wp-includes/blocks.php
@@ -261,4 +261,18 @@ function _recurse_do_blocks( $blocks, $all_blocks ) {
 	$rendered_content = preg_replace( '/<!--\s+\/?wp:.*?-->/m', '', $rendered_content );
 
 	return $rendered_content;
-}
\ No newline at end of file
+}
+
+/**
+ * Returns the current version of the block format that the content string is using.
+ *
+ * If the string doesn't contain blocks, it returns 0.
+ *
+ * @since 5.0.0
+ *
+ * @param string $content Content to test.
+ * @return int The block format version.
+ */
+function block_version( $content ) {
+	return has_blocks( $content ) ? 1 : 0;
+}
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index 3fbf169b76..c2b707dfa0 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -1509,10 +1509,11 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 
 		if ( in_array( 'content', $fields, true ) ) {
 			$data['content'] = array(
-				'raw'       => $post->post_content,
+				'raw'           => $post->post_content,
 				/** This filter is documented in wp-includes/post-template.php */
-				'rendered'  => post_password_required( $post ) ? '' : apply_filters( 'the_content', $post->post_content ),
-				'protected' => (bool) $post->post_password,
+				'rendered'      => post_password_required( $post ) ? '' : apply_filters( 'the_content', $post->post_content ),
+				'protected'     => (bool) $post->post_password,
+				'block_version' => block_version( $post->post_content ),
 			);
 		}
 
@@ -2062,6 +2063,12 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 								'context'     => array( 'view', 'edit' ),
 								'readonly'    => true,
 							),
+							'block_version' => array(
+								'description' => __( 'Version of the content block format used by the object.' ),
+								'type'        => 'integer',
+								'context'     => array( 'edit' ),
+								'readonly'    => true,
+							),
 							'protected'       => array(
 								'description' => __( 'Whether the content is protected with a password.' ),
 								'type'        => 'boolean',
diff --git a/wp-includes/version.php b/wp-includes/version.php
index dd7a769c7d..d4a950a492 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-43769';
+$wp_version = '5.0-alpha-43770';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.