We previously obtained a chimeric Friend murine leukemia virus (FMLV) envelope protein (Env) in which the whole receptor-binding domain (RBD) was replaced with a surface domain of human CD4. Here, we examined if the postbinding fusion function of the CD4-Env chimera still remains to be intact. While a pseudotype MLV bearing CD4-Env showed no infectivity, NIH 3T3 cells could be infected with a pseudotype MLV bearing both CD4-Env and a mutant FMLV Env defective in postbinding fusion function. The pseudotype MLV showed no infectivity on HeLa cells but on the FMLV receptor (mCAT1)-expressing HeLa cells. In NIH 3T3 cells, the R-peptide-deleted CD4-Env could not induce syncytia by itself but did so in co-operation with the fusion-deficient Env. Syncytia induced by the coexpression were not observed in HeLa cells but in the mCAT1-expressing HeLa cells. These results indicate that the CD4-Env could contribute postbinding fusion function in the membrane fusion process triggered by FMLV RBD-mCAT1 interaction.