Sera of ten dogs infected with Angiostrongylus vasorum were analyzed by immunoblotting proteins of first stage larvae (L1) and adult parasites. The molecular weights (m.w.) of the principal L1 proteins identified by IgG, IgM, IgA, and IgE were 18-118 kDa and those of the adult parasite were 28-209 kDa. The L1 proteins had not been recognized by IgG, IgM, and IgA antibodies in sera of dogs naturally infected with Toxocara canis, Ancylostoma caninum, and Dipylidium caninum, although only weakly by IgE. Adult parasite proteins were recognized by antibodies in sera of dogs naturally infected with gastrointestinal helminths. Adult parasite proteins with m.w. of approximately 51, 63, 92, and 209 kDa recognized by IgG could be used for specific diagnosis of canine angiostrongylosis.