IgG subclasses were determined quantitatively in sera from 63 Egyptian men who were infected with Schistosoma mansoni. Total and antigen-specific IgG was measured pre- and post-treatment. Total IgG subclass antibodies were determined by immunoradiometric assay (IRMA) using monoclonal antibodies (MoAbs). The anti-worm and anti-egg specific S. mansoni IgG subclass antibodies were quantitatively measured by ELISA using specific MoAbs and standards obtained by affinity chromatography. Our data show that total IgG of the patients was elevated in the range of two to three times above normal. The magnitude of increase differed markedly among the four subclasses of IgG. The IgG1, IgG2 and IgG3 concentrations were approximately two to four times higher than normal, whereas the IgG4 concentrations was 20 times normal (9000 mg/l). IgG1 and IgG4 tended to dominate the IgG subclass distribution of anti-soluble worm antigen preparation (SWAP) antibodies followed by IgG2 and IgG3. On the other hand, IgG1 and IgG2 dominated the IgG subclass distribution of anti-soluble egg antigen (SEA) antibodies. As with IgG1, IgG2 and IgG3, most IgG4 was non-specific. The role of IgG subclasses in the pathogenesis of schistosomiasis is not clear. However, the high concentration of IgG4 might act as IgE blocking antibody, possibly as anti-idiotypes that may play a role in down-regulation of the immune system when it is challenged with an excess of antigen.