Full-thickness skin grafts from either BN (Ag-B3) or WF (Ag-B2) rats were transplanted to WF recipients of the same sex. Six to seven days after grafting, recipients were challenged with isografts of a chemically induced rat colon carcinoma, NG-W1. In three of four experiments, mean challenge tumor volumes were greater after allografting than after skin isografting. Tumor incidences, however, were no different in rats after skin allograft or isograft placement. When isografts of a polyoma virus-induced sarcoma, P-W13, were used to challenge WF rats after skin grafting, tumor incidence was significantly greater in animals which has received allografts, whether or not they also had been immunized to P-W13 before challenge. Thus, in otherwise untreated inbred rats, grafting of full-thickness skin from donors differing at a major histocompatibility locus led to facilitated growth of solid tumor isografts in animals undergoing allograft rejection.