The gene for phosphoprotein (P) of CDV encodes three different proteins, P, V, and C. The P protein is involved in viral gene transcription and replication. In the present study, we produced MAbs against a unique domain of the CDV-P protein, from aa 232 to 507, and determined their antigenic sites. By immunizing BALB/c mice with the recombinant P protein-specific fragment, we obtained six MAbs. Competitive binding inhibition assays revealed that they recognized two distinct regions of the P protein. Western blot analysis and immunofluorescence assays using deletion mutants of the unique C-terminus of the CDV-P protein revealed that all MAbs recognized a central short region (aa 233-303) of the CDV-P protein. In addition, linear and conformational epitopes have been determined, and at least four antigenic sites exist in the P protein central region. Furthermore, four of the MAbs were found to react with the P protein of recent Japanese field isolates but not with that of the older CDV strains, including a vaccine strain. Thus, these MAbs could be clinically useful for quick diagnosis during the CDV outbreaks.