SUMMARY-A soft agar overlay was effective for the study of foci of cells morphologically altered in tissue culture by murine sarcoma virus (MSY). 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 signifi-cance of these results to an understanding of the kinetics of focus formation by MSV is discussed.-J Nat Cancer Inst 46: 1001-1007, 1971. THE FORMATION of focal areas of morpho-logically altered cells in tissue culture by infection with murine sarcoma virus (MSV) differs in the mouse and rat cell culture systems. In mouse embryo cells, murine leukemia virus (MLV) is required as "helper, " but in rat cells focus forma-tion can occur without MLV (1). To facilitate the selection and study of foci induced in both mouse and rat cells and to minimize spread of newly formed virus, an agar overlay method has been developed. Although hard agar overlays have been used for both the avian and murine systems (2-4), results with hard agar in our laboratory have been inconsistent. This communication describes the successful selection of MSV foci formed in mouse and rat cells maintained under a soft ionagar overlay. The characteristics of individual foci were studied particularly in reference to virus progeny production and cell growth potential.
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Jay A. Levy (1971) studied this question.