Glucocorticoid hormones (GCH) and anti-CD3 monoclonal antibodies (MoAbs) induce in mouse thymocytes and T-cell tumor lines an active process of cell death called apoptosis. Interleukins (IL), including IL- 1 and IL-2, have been reported to inhibit such apoptosis. In this study we show that IL-4 also reduced the DNA fragmentation characteristic of dexamethasone (DEX)-induced apoptosis in thymocytes. This effect, studied in both time-course and dose-response experiments, was also detected at low IL-4 concentrations (1 U/mL) and against high DEX levels (10(-7) mol/L). The effect of IL-4 was blocked by an anti-IL-4 but not by an anti-IL-1 alpha MoAb, and was thus both specific and direct. Phenotypic analysis showed that IL-4 protects predominantly CD4- CD8- and CD4+CD8- cells. Our findings suggest that intrathymic T-cell development may be influenced by IL-4.