We investigated the mechanisms by which interleukin-10 (IL-10) regulates antigen-specific hyporesponsiveness in asymptomatic microfilaremic (MF) individuals. Peripheral blood mononuclear cells from MF individuals (n = 11) were stimulated in vitro with Brugia malayi antigen (BMA) or mycobacterial purified protein derivative (PPD) in the presence of neutralizing anti-IL-10 or isotype control monoclonal antibodies. As expected, BMA stimulated little or no gamma interferon (IFN-γ) secretion in MF individuals, whereas PPD stimulated IFN-γ in all but one. Neutralization of endogenous BMA-driven IL-10 secretion led to augmentation of IFN-γ in seven of nine MF individuals (1.5- to 10-fold) and did so in a BMA-specific manner (PPD-driven IFN-γ was augmented in only two of eight MF individuals and only 1.5- to 2-fold), indicating that IL-10 downregulates type 1 responses in these individuals. Type 2 responses (IL-5 secretion) were unaffected by the IL-10 blockade. To assess whether IL-12 could reverse the type 1 downregulation observed, the effect of recombinant human IL-12 (rhIL- 12) on BMA-driven IL-5 and IFN-γ production was also evaluated. rhIL-12 augmented both BMA- and PPD-driven IFN-γ production 5- to 10-fold in six of nine MF individuals. These data demonstrate that IL-10 downregulates BMA- driven type 1 responses and that IL-12 can overcome downregulation of Th1 responses associated with MF but does so in a non-antigen-specific manner.