Mice infected with Schistosoma mansoni mount focal granulomatous responses around each ovum that deposits in the liver and intestinal wall. The granulomas ultimately destroy the ova while absorbing the released toxic, injurious agents. The granulomas contain T cells and other cell types, all of which are under control. For example, T lymphocyte proliferation in situ within the granulomas is probably restrained by various regulatory mechanisms. Granuloma eosinophils make VIP, and granuloma T cells have VIP receptors. Yet, the function of VIP within the granulomatous response is unknown. We studied the effect of VIP on granuloma and splenic T cell proliferation in response to Con A or soluble egg antigens (SEA). [3H]Thymidine incorporation was used to assess the rate of proliferation. VIP decreased Con A- or SEA-induced, T lymphocyte proliferation. Suppression of proliferation was most evident for T cells stimulated submaximally with mitogen or antigen. Since T lymphocyte proliferation in response to antigen or mitogen requires soluble lymphokines, we investigated the capacity of VIP to alter the expression of several lymphokines as a possible mechanism for mediating suppression of T cell proliferation. VIP decreased IL-2 production, but did not effect IL-5 or IFN-gamma release. The effect of VIP on IL-2 production was dependent on the presence of a CD4+ T lymphocyte subset. VIP could no longer modulate lymphocyte proliferation if exogenous rIL-2 was added to the cultures. The addition of neutralizing anti-IL-2 mAb, but not anti-IL-4 mAb, substantially decreased granuloma lymphocyte proliferation in response to antigen or mitogen. This suggested that granuloma T cell proliferation required endogenously produced IL-2. These findings suggest that VIP may help modulate granuloma T cell proliferation through regulation of IL-2 production.