Index: src/wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- src/wp-includes/class-wp-xmlrpc-server.php	(revision 26232)
+++ src/wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -478,6 +478,12 @@
 		$blogs = (array) get_blogs_of_user( $user->ID );
 		$struct = array();
 
+		$primary_blog_id = 0;
+		$active_blog = get_active_blog_for_user( $user->ID );
+		if ( $active_blog ) {
+			$primary_blog_id = (int) $active_blog->blog_id;
+		}
+
 		foreach ( $blogs as $blog ) {
 			// Don't include blogs that aren't hosted at this site
 			if ( $blog->site_id != get_current_site()->id )
@@ -488,9 +494,11 @@
 			switch_to_blog( $blog_id );
 
 			$is_admin = current_user_can( 'manage_options' );
+			$is_primary = ( $blog_id == $primary_blog_id );
 
 			$struct[] = array(
 				'isAdmin'		=> $is_admin,
+				'isPrimary'		=> $is_primary,
 				'url'			=> home_url( '/' ),
 				'blogid'		=> (string) $blog_id,
 				'blogName'		=> get_option( 'blogname' ),