The immunogenicity of single, private H-2 specificities has been tested. In most cases, the specificity was known to be confined either to H-2K or to H-2D. This was accomplished by the appropriate choice, as donor and recipient, or parent of the F(1) hybrid recipient, of congenic strains differing at H-2 only, and of recombinant haplotypes in donor and/or recipient. By nearly all tests, the H-2K antigen appeared to be a stronger immunogen than the H-2D antigen. Skin grafts with an H-2K difference showed median survival times (MST) of 9.3-14.5 days; for H-2D the values were 13.8-18.6. The difference was also present, though narrowed, for second-set grafts. H-2K grafts regularly engendered a demonstrable cytotoxic antibody response; with H-2D differences the response was absent or very weak. K end cytotoxic titers after multiple immunizations with lymphoid tissues ranged from 1/32 to 1/2,048, D end titers from 0 to 1/128. Hemagglutination titers showed no clear difference. The results of passive enhancement of skin grafts with H-2 alloantibody produced in donor recipient combinations, identical to those used for the skin grafts showed a different pattern. H-2K, despite its greater immunogenic strength was more easily enhanced than H-2D. Prolongation of MST's for H-2K was 2.4-6.7 days, for H-2D grafts, 0.2-1.5 days.