The viruses used in this study were nine prototype strains of APMV serotypes 1 to 9: APMV-1 (NDV), lentogenic strain LaSota/46 and mesogenic strain Beaudette C (BC); APMV-2, APMV-2/Chicken/California/Yucaipa/56; APMV-3, APMV-3/ PKT/Netherland/449/75; APMV-4, APMV-4/duck/Hon-gKong/D3/75; APMV-5, APMV-5/budgerigar/Kunitachi/74; APMV-6, APMV-6/duck/HongKong/18/199/77; APMV-7, APMV-7/dove/Tennessee/4/75; APMV-8, APMV-8/goose/ Delaware/1053/76; and APMV-9, APMV-9/duck/New York/ 22/1978. All of the viruses were grown in 9-day-old embryonated specific-pathogen-free (SPF) chicken eggs inoculated by the allantoic route, except for APMV-5, which was grown in Vero cells. The ability of viruses to produce plaques was tested on Vero and DF1 cells under 0.8% methylcellulose overlay. Exogenous protease was supplemented into the cells for replication of APMV-1 LaSota, APMV-3 and -9 (10% allantoic fluid) and APMV-8 (1 mg/ml of acetyl trypsin) (4, 9, 10) . The monolayers were fixed with methanol and plaques were visualized by immunoperoxidase staining using virus specific antiserum raised against N protein.