Abstract Monoclonal antibodies reactive with the major surface glycoprotein (G-protein) of vesicular stomatitis virus serotypes Indiana and New Jersey (VSV-Ind, VSV-NJ) have been isolated and characterized. The reactivity of each monoclonal was determined by enzyme-linked immunosorbent assay (ELISA), competitive binding assay (CBA), and the ability to neutralize infectivity. It was found that the majority of the antibodies were of the IgG 2a subclass. In the CBA, unlabeled monoclonal antibodies were used to compete for radiolabeled antibodies in binding to solid-phase immunoadsorbents. The VSV-NJ G-protein appears to contain four nonoverlapping epitopes by these analyses. However, the VSV-Ind G-protein is more complex since four epitopes were defined which exhibited varying degrees of overlap. In some cases, this overlap was defined by complete reciprocal competition between antibodies with different reactivity patterns. In other instances, partial or nonreciprocal competition between antibodies was observed. These results may indicate epitopes in close proximity or suggest allosteric modifications in the G-protein induced by antibody binding. A fifth epitope on the Ind G-protein was defined by a monoclonal antibody which could bind to the G-proteins of both VSV-Ind and VSV-NJ but could only neutralize infectivity of the VSV-Ind serotype.