The occurrence of the nucleo-, phospho-, matrix, fusion, and hemagglutinin proteins of the canine distemper virus (CDV) was investigated immunocytochemically in the brains of 3 dogs, 6 stone martens, 1 polecat, and 1 weasel. In addition, viral protein expression was studied in primary brain cell cultures of the 3 dogs after co-cultivation with Vero cells. Immunohistochemically, only minor differences, restricted to the H-4 epitope, were noted between the various species and CDV isolates. The data presented indicate that the mustelid virus is antigenically not distinct from the canine morbillivirus.