The continuous culture of a hamster melanoma cell line has led to the spontaneous appearance of a retrovirus (HaRV) with typical type-C characteristics. The virus difers from all other known hamster viruses in its ability to transform murine as well as rat and hamter cells with apparent one-hit kinetics. Guinea pig, human and feline cells were not transformed although reverse transcriptase activity was detected in the supernatant from infected human cells. HaRV-tansformed hamster embryo cells produced solid tumors (all non-pigmented) in 4 out of 35 animals when injected into hamsters while HaRV-transformed murine cells produced no tumors in mice. Injection of HaRV alone in hamsters, mice and rabbits did not induce tumors. HaRV possesses a 70S RNA which dissociates to 35S in DMSO and has a reverse transcriptase which utilizes the 70S virus RNA as a template. The size, morphology and density (1.15 g/ml) are similar to other known type-C viruses. Polyacrylamide gel electrophoresis indicates the presence of polypeptides analogous to those found in other type-C viruses.