Adult schistosomes can be transferred surgically from donor C57BL/6 mice to the portal veins of naive recipients with complete success. This procedure bypasses larval development and the antibody response of the host is directed against, and can be used to identify, those antigens released only by viable, mature parasites. Serum collected from the recipient mice (WTS) was used in Western blotting studies to probe fractionated parasite protein. Twelve immunodominant proteins were identified, ranging in molecular weight from 14 to 208 kDa. The magnitude of the IgG response against each antigen could be divided into 2 categories, on the basis of optical densitometry of the blots. In addition, defined parasite fractions were probed with WTS by Western blotting, in order to determine the relative abundance and distribution of each antigen in schistosome tissue. To confirm and expand on these initial observations, oligospecific polyclonal antibody for each immunogen was affinity purified from Western blots; it was then used in immunocytochemistry to identify the sources of secretion for 8 of the 12 antigens, at the cellular level. From the results, it appeared that after the transfer of adult worms, the first antibodies detected were mostly directed against the gastrodermis. At later times additional reactivity was expressed against the tegumental membrane. These differences probably reflect the relative abundances of the gut and tegumental secretory products.