Two mixtures of either allogeneic PARA-adenovirus 12 tumor cells or adenovirus (Adeno) 12 tumor cells and SV40 tumor cells in an equal number of 1 multiplied by 10(7) cells were transplanted subcutaneously at two sites of 10 young hamsters. The former two tumor cells were transplantable in nearly half of the transplants and the latter tumor cells in all of the transplants. By the mixture of PARA-Adeno 12 and SV40 tumor cells, 10 tumors were grown, 7 of which were mixed with Adeno and SV40 types and 3 were solely SV40 type. The other 10 tumors produced by the mixture of Adeno 12 and SV40 tumor cells revealed SV40 type only. Morphologically, Adeno type resembled the tumor induced by Adeno, and SV40 type by SV40. The results suggest that the existence of SV40 genetic information in Adeno type tumor cells is indispensable to the formation of mixed tumor in allogeneic tumor cell system. The similarity between the tumors developed by the mixed cell transplantation and those induced by PARA-adeno 7 was discussed both viro-immunologically and morphologically.