SRS 19-6 MuLV is a murine retrovirus originally isolated in mainland China. A noteworthy feature of this virus (referred to as SRS MuLV here) induces tumors of multiple hematopoietic lineages, including myeloid, erythroid, T-lymphoid and B-lymphoid. To identify the determinants of disease specificity, chimeras between SRS and Moloney MuLV (M-MuLV) were generated by molecular cloning, and the pathogenic properties of the chimeras were investigated. The results indicated that, while the M-MuLV LTR can confer lymphoid specificity to SRS MuLV, the SRS LTR by itself was not sufficient to confer multiple lineage tumorigenesis to M-MuLV; additional sequences in gag or pol were also required. Thus, a secondary determinant for myeloid/erythroid leukemia in SRS MuLV is located in gag-pol. In these chimeras, an independent determinant for T-lymphoma was found in M-MuLV gag-pol. It was also interesting that insertion of M-MuLV env into SRS MuLV decreased the rate of leukemogenicity, while insertion of SRS env into M-MuLV (SEM) accelerated leukemogenesis. The enhanced pathogenicity of SEM was found to correlate with earlier formation of MCF recombinants. The basis for the accelerated MCF recombinant formation was investigated. The endogenous polytropic MuLV env sequences contributing to several SEM MCF recombinants were identified, and the cross-over points were identified. While no obvious differences in the relative homologies between SRS MuLV env and polytropic env vs. M-MuLV and polytropic envs suggested a reason for the more rapid MCF recombinant formation, an overlapping but different set of polytropic env proviruses were found to participate in MCF formation for M-MuLV vs. SEM. Thus, the mechanisms for MCF formation appear to differ for M-MuLV and SEM.