A cat kidney cell line, CRFK-F2, was successfully inoculated in suspension and in monolayer culture with a purified mouse mammary tumor virus derived from Rill milk. The virus produced by the infected cells was identified by immunofluorescence, electron microscopy, and RNA-directed DNA polymerase assays; it was a B-type virion that did not cross-react with mouse or feline leukemia-sarcoma viruses, had spikes on its envelope, and had a RNA-directed DNA polymerase reaction that was typical of mouse mammary tumor virus. The producing cells were identified as cat cells by chromosome number, cytotoxic assays, and isoenzyme migratory patterns. A standardized method for the in vitro inoculation of cat cells is described that presently permits highly reproducible results. For the first time, the mouse mammary tumor virus is seen replicating in cells from another species, thus offering an opportunity to study the kinetics of infection of that virus.

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Supported by USPHS Grants CA-08515 and TO 1-CA-11405, Contract 1-CP-33339 from the National Cancer Institute, Contract 1-GM-3-2112 from National Institute of General Medical Sciences, and Grant in Aid M-43 from the State of New Jersey.

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