To compare the antigenic and functional domains of the matrix (M) proteins of vesicular stomatitis virus (VSV) serotypes Indiana (VSV-Ind) and New Jersey (VSV-NJ), deletion mutants and chimeras were cloned in pBSM13 and expressed as in-frame lacZ fusion proteins in Escherichia coli. Non-cross-reactive monoclonal antibodies directed to the two antigenically distinct M proteins were tested by Western blot analysis to map three epitopes of VSV-Ind M protein and four epitopes of VSV-NJ M protein. Epitope 1 of the VSV-Ind M protein and epitope II of the VSV-NJ M protein both mapped to the highly basic N-terminal 34 amino acids of each homotypic M protein. Epitopes 2 and 3 of the VSV-Ind M protein and epitopes III and IV of the VSV-NJ M protein mapped to a region spanning amino acids 35 to 74. Epitope I of the VSV-NJ M protein mapped to a region between amino acid 75 and the C terminus. The similarity in location of the serotypically unique antigenic determinants of the two M proteins suggested that they may have a common functional domain. This hypothesis was substantiated by the finding that the two M proteins and various chimeras expressed in CV-1 cells by a recombinant vaccinia virus system were able to rescue M gene temperature-sensitive mutants of both VSV serotypes.