A method is described which obviates the use of H 2 recombinant strains for production and detection of Ia antibodies. The antibodies are produced by immunization with spleen cells in a strain combination in which the donor differs from the recipient either in the K end of the H 2 complex or in the whole complex. If the right schedule is used, this immunization produces antibodies against both H2 and Ia antigens. The H 2 antibodies are absorbed out with erythrocytes or T lymphocytes rendering the antiserum specific for Ia antigens. The presence of Ia antibodies is asserted by determining the tissue distribution of the antigens detected with the absorbed antiserum. Another antiserum is then prepared in the same strain combination by immunization with tissue that does not contain Ia but does contain H 2 antigens (e.g., a sarcoma of the proper genotype). The second antiserum contains only H 2 antibodies and the presence of these antibodies is again asserted by determining the tissue distribution of the corresponding antigens. The molecular distinctiveness of the putative Ia and H2 antigens is then demonstrated by the newly developed technique of antibody mediated induction of resistance to cytotoxicity (lysis). If the antigens detected with the two antisera move independently in the cell membrane, and if the antigens detected with the first antiserum do indeed have Ia like properties, it is concluded that the antiserum detects Ia antigens. This method should prove to be useful for the detection of Ia antigens in H 2 haplotypes for which no intra H 2 recombinants are known, and for the detection of Ia like antigens in other mammalian species, particularly in man.