In this study we neutralized endogenous IL-10 in PBMC from individuals chronically infected with Schistosoma haematobium by using anti-IL-10 MoAbs, and examined the effect and adult worm antigen (AWA)-specific responses in both fresh or cryopreserved cells. Anti-IL-10 alone increased background proliferation of PBMC, but did not augment the AWA-specific responses in either fresh or frozen cells. In freshly isolated PBMC, IFN-gamma production in response to AWA was enhanced significantly in the presence of anti-IL-10. In cryopreserved cells, the augmentation of IFN-gamma in the presence of anti-IL-10 was four-fold less than in the freshly-isolated cells. Neutralization of IL-10 had no effect on IL-4 production. These data show that IL-10 plays a role in specifically down-regulating Th1-but not Th2-type responses and, in contrast to previous reports, anti-IL-10 does not augment proliferation to parasite antigen in chronic schistosomiasis.