Nucleotide sequences encoding gp70, Prp15E, and the U3 region of the long terminal repeat (LTR) distinguish mink cell focus-forming (MCF) retroviruses that can induce leukemia in AKR mice from closely related MCF and ecotropic murine retroviruses that are nonleukemogenic in all inbred mouse strains tested (Lung et al., Cold Spring Harbor Symp. Quant. Biol. 44:1269-1274, 1979; Lung et al., J. Virol. 45:275-290, 1983). We used a set of recombinants constructed in vitro from molecular clones of leukemogenic MCF 247 and nonleukemogenic ecotropic Akv to separate and thereby directly test the role of these genetic elements in disease induction. Leukemogenicity tests of recombinants in AKR mice show that introduction of fragments containing either an MCF LTR or MCF gp70 coding sequences can confer only a very low incidence of disease induction on Akv virus, whereas an MCF type Prp15E alone is completely ineffective. Recombinants with an MCF 247 LTR in combination with MCF Prp15E are moderately oncogenic, whereas those with an MCF 247 LTR plus MCF gp70 coding segment are quite highly leukemogenic. Mice infected with the latter virus show a substantial increase in latent period of disease induction relative to MCF 247; this delay can be reduced when Prp15E, and hence the entire 3' half of the genome, is from MCF 247. Surprisingly, sequences in the 5' half of the genome can also contribute to disease induction. We found a good correlation between oncogenicity and recovery of MCF viruses from thymocytes of injected mice, with early recovery and high titers of MCF in the thymus being correlated with high oncogenicity. This correlation held for recombinants with either an MCF or ecotropic type gp70. Together, these results (i) demonstrate that at least four genes contribute to the oncogenicity of MCF viruses in AKR mice and (ii) suggest that recombinants with only some of the necessary MCF type genes induce leukemia because they recombine to generate complete MCF genomes. Although neither Akv nor MCF 247 is leukemogenic in NFS mice, recombinant viruses whose gp70 gene was derived from Akv but whose LTRs were derived from MCF 247 induced a low incidence of leukemia in this mouse strain.