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A new class of murine leukemia viruses, isolated from wild Mus musculus trapped in California, is described. These viruses, designated "amphotropic," replicate in mouse, rabbit, mink, human, guinea pig, and rat cells, but not in hamster, quail, or duck cells. They show N-tropism for mouse cells, and do not trigger the XC cell response. They are distinct by interference and virus neutralization testing from the previously recognized mouse-tropic and xenotropic MuLV classes. Mouse-tropic viruses occuring along with three of the four amphotropic isolates were found to be distinguishable by virus neutralization from other mouse-tropic murine leukemia virus strains of laboratory mouse origin.

Seven murine leukemia virus field isolates (uncloned) from wild mice (Musmusculus) of four widely separated areas in southern California show an unusually wide in vitro host range. They replicate well in human, feline, canine, guinea pig, rabbit, rat, and mouse cells, whereas bovine, hamster, and avian cells are resistant. Since this host range includes that of both mouse tropic (ecotropic) and xenotropic murine leukemia viruses, they are designated as "amphotropic". No purely xenotropic virus component is detectable in these field isolates. They may represent the "wild" or ancestral viruses from which the ecotropic and xenotrophic murine leukemia virus strains of laboratory mice have been derived.