Seven-week-old female Syrian golden hamsters (Mesocricetus auratus) showed different degrees of susceptibility to Schistosoma mansoni, as assessed by the percentage of cercariae recovered as adult worms 6 weeks after infection. Plasma of the low (A), medium (B) and high (C) susceptibility groups were tested immunochemically. No differences were observed in the concentrations of albumin, alpha 1-, alpha 2-, beta- and gamma-globulins as measured by cellulose acetate electrophoresis. However, a significantly higher percentage of animals in groups A and B than in group C had an S. mansoni specific "beforked" IgG precipitin band and specific antibodies against a worm tegumental antigen preparation (AWT). Conversely, more animals in group C made antibodies against a "denuded" worm-body antigen preparation (AWB) than in groups A and B. However, by using the enzyme-linked immunosorbent assay, no significant differences in antibody titres against AWT, AWB and a total worm antigen (AVA) were observed in the animals in groups A, B and C. Upon consideration of the immunochemical data in relation to the distribution pattern of susceptibility to infection, we propose that the intensity of S. mansoni infection in the hamster is a polygene-controlled phenomenon and depends upon the presentation of differing parasite antigenic component(s) to the host.