We assessed the effect of anti-CD3-stimulated secretion of cultured human Th1- and Th2-like cells on leukotriene C₄ (LTC₄) secretion in isolated human eosinophils. T helper (Th) cell subsets were generated from human naive CD4⁺ T cells cocultured with irradiated human transformed B cells and either recombinant human interleukin (rhIL)-1 plus rhIL-6 plus rhIL-12 for Th1-like cells or rhIL-1 plus rhIL-6 plus rhIL-4 for Th2-like cells. Coincubation of eosinophils with 1:5 dilution of Th2-supernatant (Sup) caused an increase in LTC₄ secretion caused by 0.1 µM formyl-Met-Leu-Phe and 5 µg/ml cytochalasin B from 921 ± 238 to 3,067 ± 1,462 pg/10⁶ eosinophils (P < 0.01). Th1-Sup at the same dilution had no augmenting effect on stimulated secretion of LTC₄ in eosinophils despite substantial concentrations of granulocyte-macrophage colony-stimulating factor (GM-CSF) in the supernatant. Dilution of Th1-Sup caused increased LTC₄ that returned to baseline after immunoabsorption of GM-CSF, suggesting the presence of a possible inhibitory factor. We demonstrate that pretreatment of eosinophils with 1:5 dilution of Th2-Sup but not of Th1-Sup causes substantial augmentation of LTC₄ secretion in vitro and establishes that human Th2 cells cause direct augmentation of LTC₄ secretion within 15-30 min of exposure.