The segregation of IgG2a and IgG1 immunoglobulin isotypes as markers for Th1 and Th2 lymphocytes respectively, was investigated in mice exposed to normal or optimally-irradiated S. mansoni cercariae. Using a panel of ELISAs, soluble antigens from lung-stage schistosomula, adult worms, or eggs, were probed with serum samples collected at biweekly intervals. Infected mice developed increased IgG1 responsiveness to all three antigens, especially between weeks five and seven, whereas IgG2a responses were lower, particularly to egg antigens. This confirms that Th2 responses are dominant after the onset of patency in infected mice. In comparison, vaccinated mice developed lower levels of IgG1, and higher levels of IgG2a to larval and worm antigens. Thus, they had balanced expression of IgG1 and IgG2a, despite having a dominant Th1 lymphocyte population. An elevated IgG1 response to egg antigens in vaccinated mice challenged with normal parasites, occurred two weeks later than in normal mice. Mice exposed to male-only cercariae developed IgG1 and IgG2a antibodies to larval and worm antigens. However, they also had elevated IgG1 to egg antigens from week five, despite a total absence of eggs. Therefore, adult worm antigens may cross react with the egg and stimulate the switch to Th2 dominated responsiveness.