The AKR mouse strain is uniquely susceptible to thymic lymphoma. Friend murine leukemia virus (F-MuLV) induced thymic lymphoma in this strain after a median latency of 3-4 months, whereas in several other mouse strains F-MuLV caused erythroblastosis. In the investigation of genetic determinants of the AKR predisposition to thymic lymphoma, series of congenic mouse strains and progeny of crosses involving AKR mice were inoculated with F-MuLV; these mice were followed for the development of lymphoma. The results suggested that AKR genes on chromosomes 7 and 15, other than inherited ecotropic virus, are involved in the predisposition to rapid onset of thymic lymphoma after inoculation with F-MuLV.