The structural polypeptides of two strains of canine distemper virus and the Lec strain of measles virus were analysed by SDS-polyacrylamide-slab-gel electrophoresis. One strain of canine distemper virus derived from a live vaccine (Convac, Dumex), contained six major structural polypeptides with mol.wt. of 85, 78, 59, 43, 41 and 34 x 10(3). The 85K polypeptide was glycosylated. It was interpreted to be equivalent ot the 79K glycoprotein of the measles hemagglutinin. The second strain, a rapidly growing variant of the Onderstepoort strain of canine distemper virus characterized by extensive syncytium forming cytopathic effects in tissue culture, contained the 5, 43, 41 and 34K polypeptides, but the 85 and 78K polypeptides were not present in detectable amounts. The 43K polypeptide was identified as cellular actin by limited proteolysis. By use of monospecific rabbit hyperimmune sera against each of the major structural polypeptides of measles virus, the 59, 41 and 34K structural polypeptides could be identified as nucleocapsid protein (NP), fusion (F) polypeptide, and the membrane (M) polypeptide, respectively. In neutralization tests with rabbit hyperimmune sera against each of the two strains, this Onderstepoort strain, which contained reduced amounts of the hemagglutinin glycoprotein, gave higher neutralization titers than the vaccine strain.