Null types of 12 human erythrocyte blood groups are reviewed. They have helped in identifying new antigens and defining the various genetically-distinct systems. They are very valuable in identifying the antibodies in alloimmunized people and in transfusion therapy of some of these people. Fy(a-b-) erythrocytes resist invasion by malarial parasites. At least two (Rh null and the McLeod type) are responsible for congenital hemolytic disorders. Testing for K15(Kx) on neutrophils appears to be diagnostic for chronic granulomatous disease of the sex-linked recessive type.