The cellular and humoral immune responses of patients with S. mansoni infection were evaluated before and one month after each of two intramuscular doses of diphtheria/tetanus toxoid vaccine. Patients were divided into "responder" and "non-responder" groups based on anti-tetanus toxoid (anti-TT) IgG levels after vaccination. The specific anti-TT IgG1 response of the responder group was predominantly in the IgG, subclass. The lymphoproliferative response to PHA was also significantly higher in the responder group; this elevation was detectable before and after each vaccination. The responses to PWM and SPL were comparable in the two groups before vaccination, although the responder group had a higher response to SPL after vaccination. IgG antibodies for schistosome adult worm and egg antigens were significantly lower in the responder group prior to vaccination but not thereafter. Anti-diphtheria IgG antibodies were comparable in the two groups after vaccination at all times. Clinically, the non-responder patients had a higher incidence of splenomegaly (84.6% vs 44.8%) and were significantly older than the responder patients (mean 34.1 yrs vs 18.7 yrs). The cause for the reduced anti-tetanus IgG response in schistosomiasis patients is believed to be multifactorial. T cell or antigen presenting cell dysfunction, high levels of IgG antibodies specific for schistosome antigens, splenomegaly and age are factors that might lead to reduced anti-TT IgG response.