To investigate the relationship between cytokine production and the increased levels of serum IgE and peripheral eosinophilia commonly accompanying human helminth infections, we studied the ability of PBMC of normal (Nl) (n = 18) and eosinophilic individuals with helminth infections (Hl) (n = 9) to produce IL-3, IL-4, IL-5, granulocyte-macrophage-CSF, and IFN-γ in vitro after stimulation with PMA (50 ng/ml) and ionomycin (1 μg/ml). The two groups differed in both the levels of serum IgE and eosinophilia. For mitogen-induced production of granulocyte-macrophage-CSF and IFN-γ, there was no difference in cytokine production between the two groups. In marked contrast, supernatants from PBMC of infected individuals had significantly higher levels of IL-4 (mean = 213 pg/ml for Nl and 944 pg/ml for Hl, p < 0.02), IL-5 (mean = 180 pg/ml for Nl and 1118 pg/ml for HL, p < 0.001), and IL-3 (mean = 13900 pg/ml for Nl, 28029 pg/ml for Hl, p < 0.05). In addition, helminth-infected patients had ~5-fold greater numbers of T cells capable of producing IL-5 and 2.5-fold greater frequency of IL-4- secreting cells than did normal individuals; GM-CSF-and IFN-γ-producing T cell numbers were not significantly different in the two groups. IL-3- producing cell frequencies could not be evaluated by this method. There was a direct correlation between IL-4 production and IL-5 production at the level of both protein production and frequency of T cells capable of producing these cytokines. These data indicate that individuals with reactive eosinophilia and elevated serum IgE have an expanded population of lymphocytes producing IL-4 and IL-5 and the association of the two suggests that the regulation of IL-4 and IL-5 may be linked.