Mason-Pfizer monkey virus (MPMV) and PMFV, an isolate from a human continuous cell line, were compared by Sepharose bead immunofluorescence assay. According to the results with p27-specific assays the main structural protein of both viruses seems to be identical in the prominent antigenic determinants. Differences were found when comparing the p15s indicating that this viral protein contains type-specific antigenic determinants.