Two variants of the spleen focus-forming virus (SFFV), SFFVP and SFFVA, induce acute erythroleukemia in mice but differ in their effects on erythroid cells as well as in the posttranslational modification of the product of their envelope genes. To localize the region of the SFFV envelope gene responsible for these differences, we utilized a recombinant virus containing the 3' half of the SFFVP env gene, where the vast majority of the differences between SFFVP and SFFVA reside, and the SFFVP long terminal repeat (LTR) on an SFFVA background. Analysis of the recombinant virus indicates that it is capable of inducing all of the biological effects previously associated with SFFVP, including the ability to proliferate and differentiate without the need for erythropoietin. In addition, the env gene product of the recombinant virus can be detected on the cell surface, a property previously associated only with SFFVP. Although the recombinant virus also contains LTR sequences from SFFVP, we do not believe it is likely that the four LTR nucleotides that are unique to SFFVP are responsible for the biological or biochemical differences observed. These results strengthen the argument that the SFFVP env gene product acts at the cell surface to alter the hormonal requirements for erythroid cell growth and differentiation.