Non-transforming helper viruses were isolated from pools of two hamster-specific sarcoma viruses in which isolations by terminal dilution procedures had previously been unsuccessful. This presumably resulted from approximately equivalent levels of sarcoma and helper viruses in the original pools. Helper virus was isolated from morphologically normal cells selected from tissue culture plates showing relatively few transformed colonies, and by end-point dilution from cloned transformed cells. Focus-formation by the hamster-specific sarcoma viruses was independent of the spread of virus and non-producer cell lines carrying the sarcoma virus genome were isolated; thus these viruses were apparently unable to replicate independently of helper virus. Hamster-specific virus antigens were not detected in hamster cells infected productively or non-productively with a murine sarcoma virus; thus, evidence for virus activation by the sarcoma virus genome was not obtained.


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