IL-4 production is selectively impaired in Vav1^–/– Th2 cells. Naive CD4⁺ T cells from Vav1^–/– or wild-type mice were primed in anti-CD3/CD28–coated plates under Th2-inducing conditions in the presence of an anti–IFN-γ antibody. After 7 days, the cells were washed, counted, and restimulated at 2 × 10⁵ cells/well with plate-bound anti-CD3 antibody. Proliferation was determined after 72 hours of stimulation by ³HTdR uptake. Levels of IFN-γ, IL-2, IL-4, IL-5, or IL-13 in 48-hour stimulated culture supernatants were quantified by ELISA. All data are expressed as the mean value of triplicate determinations ± SE. The data shown are representative of 3 independent experiments.