We investigated the genetics of transplanting a NEDH rat insulinoma in various donor-recipient combinations. The insulinoma survived indefinitely when it was transplanted in the NEDH strain. Mean survival times of 8.6 +/- 1.2 and 3.3 +/- 1.0 days were recorded in allogeneic and xenogeneic combinations, respectively. No prolongation of mean survival time was found when the rat insulinoma was first passed through the athymic nude mouse and then transplanted as either an allograft or xenograft. The findings of the major histocompatibility antigen (Ag-B7) on the original insulinoma and the tumor that had been passed through the nude mouse demonstrates no demonstrable loss of transplantation antigens during passage.