Isogenic male skin grafts transplanted to C57BL female multiparas, mated within the strain, did not show prolonged survival as consistently as in the experiments of Breyere and Barrett. Those multiparas that did accept male skin grafts for prolonged periods also tolerated second and third male skin grafts for a longer time. Our purpose was to test whether the Breyere and Barrett phenomenon is caused by enhancement. This was suggested by the results of the two following experiments. Nulliparous C57BL mice that had been in parabiosis with C57BL multiparas did reject male skin grafts at a later time than C57BL virginal controls. Furthermore, male skin grafts showed extended survivals when grafted to virginal mice that were injected with serum from multiparous mice. Transfer of enhanced male skin grafts from multiparas to adult nulliparous females did result in an accelerated rejection. The median survival time of 15 transferred C57BL/6 male skin grafts was 17.6 days rather than the median survival time of 26.0 days expected of normal skin. Transfer of one of two simultaneously placed male skin grafts that both had shown enhancement in multiparas also resulted in earlier rejection of the transferred graft, whereas the graft that remained on the multiparous recipient was rejected significantly later. These observations do not seem to support the concept that immunological enhancement is a consequence of a peripheral effect.