Abstract
A soft agar overlay was effective for the study of foci of cells morphologically altered in tissue culture by murine sarcoma virus (MSV). All foci in mouse embryo cells yielded both sarcoma and leukemia viruses, but the altered cells could not be propagated. Many foci in rat cells, on the other hand, did not release either virus unless superinfected by leukemia virus; all the foci tested could be grown as independent cultures. The significance of these results to an understanding of the kinetics of focus formation by MSV is discussed.

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