When cell suspensions from people with serum protein abnormalities or strongly positive direct antiglobulin tests are tested against Lorne IgG Rh Blood Grouping Reagents they may produce false positive reactions. This is due to the fact that these reagents are formulated with potentiators and contain high protein levels to enable the IgG antibodies to react in direct agglutination tests. The use of Rh-Hr Control Serum enables such false positive results to be recognised. A positive result with Rh-Hr Control Serum invalidates the results obtained with these Rh Grouping Reagents.