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THE origin of the murine sarcoma viruses (MuSV) found in tumours resulting from the injection of mice with leukaemo-genic viruses (MuLY)1–3and from spontaneous tumours3,4is very obscure. Viruses isolated from spontaneous tumours3,4might have been induced through accidental horizontal transmission of Moloney sarcoma virus. The origin of isolates of Moloney sarcoma virus1,2and Kirsten sarcoma virus5is also unknown because the virus preparations used had been passaged through rats2,5or mice of various strains for many years1. Nor is it known whether sarcoma virus was present in the preparations of murine leukaemia virus administered to the animals subsequently developing sarcomas. It has been suggested that Moloney MuSV is derived from Moloney MuLV6, but there may be an endogenous, latent sarcoma virus present in mice which could perhaps be activated by the administration of a leukaemogenic virus. We now report that a clone-purified MuLV, isolated from an established cell line in tissue culture and having in vitro no demonstrable content of MuSV, induced sarcomas in several strains of mice and MuSV could be readily isolated from these sarcomas. The virus was also present in the plasma of the tumour-bearing mice.