Seventeen avian paramyxoviruses consisting of ten reference strains and seven recent isolates were classified by immunodiffusion into six species on the basis of the antigenic specificity of their M protein (MP) antigens. The species were represented by the following viruses. (1) Newcastle disease virus, (2) Chicken/California/Yucaipa/56, (3) Turkey/Wisconsin/68, (4) Duck/Mississippi/75, (5) Pigeon/Otaru/76 and (6) Budgerigar/Kunitachi/1/74. The differences in the HN antigens between viruses which were classified into the same species on the basis of the antigenic specificity of their MP antigens, may provide a basis for classifying them into types.