Titration patterns of a murine sarcoma-leukemia virus complex: evidence for existence of competent sarcoma virions

Stocks of inurine sarcoma virus show titration patterns ranging from one-to two-hit kinetics. The comparison of various titrations of this virus, both with and without added helper virus, to theoretical model systems composed of defined constituents, suggests the existence of a sarcoma virus that does not need coinfectinig murine leukemia virus to be manifested as a focus-forming unit. The behavior of such nondefective particles is compatible with a postulated leukemia-sarcoma virus hybrid.