Avian pox viruses constitute a subgroup of pox viruses which include fowlpox, pigeonpox, turkeypox, canarypox, sparrowpox, juncopox and starlingpox (5). Fowlpox virus has been studied more extensively than other avian pox viruses. Morphologically, all avian pox viruses have the same ultrastructural details. Differentiation of avian pox viruses has been attempted on the basis of host susceptibility, characteristic lesions on the dermal epithelium, growth on ectodermal chorioallantoic membrane, cross-protection, cross-neutralization, complement-fixation, and agar-gel precipitation tests. In an earlier study (4), a passive hemagglutination test was developed to measure antibody response of chickens following inoculation with fowlpox virus. The present investigation was conducted to evaluate the capability of the passive hemagglutination and virus-neutralization tests in differentiating avian pox virus agents. Also evaluated were response of day-old chicks, growth in primary cells, and plaque formation. Experimental birds. Colombian-cross chickens 3-4 months old were used for inoculation of viruses. Approximately 0.2 ml of unpurified chorioallantoic membrane (CAM) suspension of each avian pox virus with Freund's adjuvant was used to inoculate a bird by feather-follicle method. Hyperimmune sera were produced by in-