Development of a method for the detection of H-2 and non-H-2 isoantigens with the leukocyte agglutination reaction is described. Non-H-2 leukocyte agglutinins were induced by sensitization with injections of living spleen and thymus cells, by pregnancy and by skin homografts. Cross reaction of non-H-2 antisera with leukocytes of many inbred mouse strains was observed, indicating that these strains share some of the non-H-2 isoantigens. Selective in vivo absorption was done with one non-H-2 antiserum and a minimum of two separate agglutinating factors was found.