Murine leukemia viruses capable of malignant transformation of mink tissue culture cells have been isolated from an AKR thymoma cell line and from a spontaneous reticulum cell sarcoma in an NIH Swiss mouse partially congenic for the AKR ecotropic virus-inducing locus Akv-2. In contrast to the recently described mink cell focus-inducing strains of murine leukemia virus, at least one of the two transforming strains is replication defective. Nonproducer mink cells carrying the genome of the transforming virus of AKR origin have been isolated, and pseudotype transforming viruses generated.