Alloimmune congenic anti-H-2 sera, dependent on the combination of H-2 haplotypes of donor-recipient strains, exert a strong cytotoxic activity against human cells. While strong sera react with almost all samples of human cells, in serial dilutions pronounced differences in strength of reactions can be found with different human cell samples in correlation with their HL-A phenotype. The strongest associations of some anti-H-2f and anti-H-2p sera were found with HL-A2 (r = 0.70 - 0.80), HL-A7 (r = 0.45 - 0.55) and HL-A27 (r = 0.65 - 0.75) (with HL-A27 in groups of patients with high frequency of this antigen r = 0.80 - 0.90). Associations with HL-A9 were also indicated. The reaction pattern itself, as well as absorption and family studies, strongly indicates that for the individual sera the differences in reaction strength are, in essence, quantitative in nature. While it is basically unclear which part of both MHS systems is responsible for the observed cross-reactions, out of several alternatives the hypothesis indicating the polymorphism of common-part structures on H-2 and HL-A molecules seems to be the most plausible.