Hamsters injected intracardiacally with wild-type SV40 developed diverse tumors, with no one type predominating. However, hamsters injected with small t deletion mutants of SV40 uniformly developed true histiocytic or B-cell lymphomas, both of which arise from rapidly dividing cell populations, with very few tumors of other types. Our results are consistent with in vitro studies which suggest a requirement for the small t function in SV40 transformation of nondividing cells.