Moloney murine leukaemia virus (MoMLV) enters host cells by membrane fusion between the viral envelope and the host cell membrane. The cytoplasmic tail (R peptide) of the MoMLV envelope protein (Env) is cleaved by the viral protease during virion maturation. R peptide-truncated Env induces syncytia in susceptible cells but R peptide-containing Env does not, indicating that the R peptide inhibits membrane fusion. To examine the function of amino acid residues at the R peptide cleavage site in virus entry, mutant Env expression plasmids containing amino acid substitutions at these cleavage site residues were constructed. Some of these mutants induced syncytia in NIH 3T3 cells, even though they expressed the R peptide, indicating the importance of these residues for membrane fusion inhibition by the R peptide. Some mutants in which R peptide cleavage was detected had comparable transduction efficiency to wild-type Env, but mutants in which R peptide cleavage was not detected had lower transduction efficiency than wild-type Env. This result strongly supports that R peptide cleavage is required for virus entry.