In the conventional focus assay for murine sarcoma virus ( Hartley and Rowe, 1966), the formation of a focus involves repeated rounds of infection, and, as is shown in this report, the possibility of alterations in the genome of the virus is thereby increased. Multiple rounds of infection were avoided by infecting cells in suspension, plating them sparsely, and allowing them to grow into colonies. XC cells were added to detect which colonies were producing leukemia virus. When cells were infected with the Moloney sarcoma-leukemia virus (M-MuSV(MuLV)), four types of colonies were seen: (1) morphologically normal with syncytia (XC +) or (2) without syncytia (XC −), (3) morphologically transformed with no syncytia, (4) transformed with syncytia. The proportions infected by MuSV (transformed cells) or by MuLV (XC +) conformed to Poisson's distribution, and this allowed the calculation of the titers of MuSV and MuLV. Clones of chronically infected cells could readily be isolated. A clone of transformed cells called G8 was derived from JLS-V9 cells infected with M-MuSV(MuLV). The cells produced no MuLV detectable by cocultivation with XC cells, but they did produce sarcoma virus detected by the production of sarcomas in mice and morphological transformation of several lines of mouse cells in culture. The virus had a density of 1.16 g/cm 3. The kinetics of focus formation were one-hit when assayed by the conventional assay. Virus picked from most ( 32 38 ) of these foci consisted of a mixture of sarcoma virus and leukemia virus but some ( 4 38 ) foci were found that produced sarcoma virus alone (presumably “competent” sarcoma virus, i.e., helper-independent). The presumed “competent” sarcoma virus was carried through 4 successive passages and each time, most of the foci were found to contain both MuSV and MuLV, but some produced MuSV only. In contrast, the original, chronically infected G8 cells did not release detectable MuLV through more than 30 passages. Leukemia virus or defective sarcoma virus segregated from the competent MuSV with low and equal frequencies only when new mouse cells were infected. Examination of the individual cells within foci formed by spread of viral infection showed that some cells produced only competent virus; other cells from the foci produced MuSV and MuLV; others were transformed nonproducers containing defective MuSV that could be rescued by superinfection with MuLV; and still others were transformed but MuSV could not be rescued from them. No evidence was found for the presence of a helper virus in excess of the concentration of sarcoma virus and competence appears to be a property of the virion itself. The data suggest that the competent MuSV is similar to the helper-independent strain of the Rous avian sarcoma virus. It is not known why this virus is negative in the XC assay, although the MuLV that segregates from it is positive in this assay. In the search for a helper virus, a form of MuSV was found that did not morphologically transform the cells it infected, nor was it produced by them, but both transformation and release of MuSV appeared on superinfection with MuLV.