Genetic control of the immune response in mice to Leishmania mexicana surface protease. Congenic mouse strains were tested in the lymphocyte proliferation assay for their response to the purified surface protease of Leishmania mexicana (gp63). The data obtained allow us to distinguish three different patterns of response, influenced both by H-2 (class II) and non-H-2 genes. Mice of the C57BL/10 (B10) background carrying H-2 haplotypes b,q, and r were found to be high responders; those carrying H-2 haplotypes d, j, v, and z were low responders; and those with H-2a, H-2f, H-2k, H-2p, and H-2u haplotypes were intermediate responders. Studies with H-2 recombinant strains indicated that the high responsiveness on the B10 background was determined by the Ab allele and the low responsiveness influenced by the Ad allele. Other genes besides H-2 appear to have a role in the immune response as shown by the fact that some strains with BALB, DBA, or C3H background differed in their pattern of responsiveness from B10 background strains carrying the corresponding H-2 haplotypes. By using recombinant protein, the influence of the leishmanial surface lipophosphoglycan that might co-purify with gp63, on the MHC restriction of the response to gp63 was excluded. The immune response to gp63 did not correlate with susceptibility of mouse strains to cutaneous infection with L. mexicana promastigotes. 