Following the infection of rats with Angiostrongylus cantonensis, the occurrence and molecular features of circulating antigens (CAs) were analyzed, with special reference to their origin in worms, by SDS-PAGE combined with an immunoblotting technique. Antisera against the CAs were obtained by immunizing rats with sera from rats 35, 91, and 150 days postinfection. The anti-sera, referred to as anti-CA(35), anti-CA(91), and anti-CA(150), respectively, formed one or two precipitin lines when tested in Ouchterlony plates against extracts of digestive organs (DE) and of reproductive organs (RE) from adult female worms. When the anti-CA(35) was used as a blotting antibody under nonreducing conditions, a set of clearly spaced narrow bands with molecular weights (mol. wt.) in the range of 90,000-180,000 daltons developed only in the case of the DE. Besides the antigen(s), additional bands with mol.wt. of 115,000 and 185,000 daltons were revealed in the case of the RE when two other antisera were used. Immunoblot analysis of the immunoprecipitates, derived from anti-CA(150) and sera of infected rats, revealed the occurrence of two types of protein as the major CAs: one had a mol.wt. in the range of 140,000-180,000 daltons and was found in the serum 14 days postinfection, and the other, with a mol.wt. of 185,000 daltons, was found in the serum 35-150 days postinfection. Immunohistochemical studies localized the CAs predominantly in the cytoplasm of both uterine eggs and maturing oocytes.