To help illuminate the surface topography of paramyxovirus nucleocapsids, epitopes recognized by monoclonal antibodies have been mapped on the primary structure of human parainfluenza virus type 1 (hPIV1) nucleoprotein (NP). Full-size NP (524 amino acids) was used, as well as a series of truncated proteins with segments resected from either their carboxyl or their amino termini. Immunoprecipitation by three anti-hPIV1 NP monoclonal antibodies required the presence of amino acids within the carboxyl-terminal 23% of NP. This was consistent with an earlier study of the closely related Sendai virus (SV) NP which mapped all epitopes to regions near the carboxyl terminus. However, in contrast to those results, we found that three other antibodies specific for hPIV1 NP recognized epitopes in the amino-terminal 30% of the molecule. Two of these antibodies also cross-reacted with SV NP and with SV nucleocapsid complexes, showing that the same epitopes were present in the SV protein and were accessible on the nucleocapsid surface. Differences in immunogenicity of these epitopes in the hPIV1 and SV nucleoproteins may reflect sequence differences elsewhere in each NP molecule. In addition, two antibodies to epitopes near the NP carboxyl terminus caused P protein to be released from SV nucleocapsid complexes and prevented binding of exogenous P protein to nucleocapsids. Antibody inhibition of P protein binding helps to locate the NP domains important for attachment of P protein during nucleocapsid assembly.