A new type of murine leukemia virus has been detected in thymuses of leukemic and late preleukemic AKR mice, in lymphomas developing in NIH Swiss mice carrying the AKR ectopic virus-inducing loci Akv-I or Akv-2, and in the thymus of a preleukemic C58 mouse. The viruses induce focal areas of morphologic alteration in a mink lung cell line and are tentatively referred to as "mink cell focus-inducing" (MCF) strains. They have the host range of both xenotropic and N-tropic ecotropic murine leukemia viruses, are neutralized by antisera to both ecotropic and xenotropic viruses, and are interfered with by both viruses. They may represent a particular type of genetic recombinant which emerges during the preleukemic period in high-ecotropic-virus mouse strains, and they may play a significant role in the etiology of spontaneous lymphomas.