The potential for interactions between different antigens in grafted tissue is important, for practical reasons, to the transplant surgeon. In addition, these interactions can provide the immunologist with an insight into complex cellmediated immune responses. Minor histocompatibility antigens are both useful and convenient for experimental examination of interaction phenomena. We have previously reported (Johnson et al. 1980) that the survival on C57BL/6By (B6) hosts of skin grafts from F1 hybrids between pairs of HW strains a set of congenic strains histoincompatible with B6 (Bailey 1975)was in most cases predicted accurately by simple additive or independent response models. To investigate further some of the cases that were no t predicted well by these models, we tested individual B6-strain hosts simultaneously with skin grafts from such F1 donors, as well as with grafts from F 1 donors produced by crossing each of the parental HW strains with the B6 strain. When the data were analyzed, it became clear that some of the (HW x B6)F 1 grafts were rejected more slowly than they had been in earlier experiments (Johnson et al. 1980). This suggested that the response to one antigen may have interfered or competed with the response to another. We now report on further grafting experiments that focus specifically on this effect. The experimental plan was to see whether there were differences between the rejection by B6 females of (HW~ x B6d')F 1 female skin when grafted alone and when grafted concomitantly with B6 male competitor grafts or (HW x B6)F 1 male competitor grafts. The two kinds of competitor graft were used to determine the effect of having the competitor antigen present on the same cells as the test antigen. Preliminary observations indicated that the effect was detected best if the recipient responded weakly to the test antigen, and if the competitor antigen evoked a relatively strong rejection. We chose H-Y antigen as the strong competitor since, in our hands, the B6 female hosts used in these experiments reject B6 male skin in about 14 days (Johnson 1979). The test antigens were chosen from those represented by congenic strains described in detail elsewhere (Bailey 1975). For convenience, a