Despite evidence for the expression of the low-affinity Fc receptor for IgE (Fc epsilon RII/CD23) in several pathological conditions, the role played by CD23 in normal human T cells is still unclear. We studied the effect of a stomal-derived cytokine, interleukin (IL-7), on the expression of CD23 in human T cells stimulated with 10 micrograms/ml phytohemagglutinin (PHA). The results demonstrate that IL-7 did not induce CD23 expression in the resting T cells. However, PHA-induced CD23 expression was enhanced by costimulation with IL-7 (1,000 U/ml). Cytofluorometric analysis revealed that CD23 expression in activated T cells was enhanced by the addition of IL-7 (from 2% to 18%). It was also observed that the effect of IL-7 on CD23 expression is exclusively on CD4+ T cells. The enhanced expression of CD23 was blocked by an anti-IL-7 monoclonal antibody (mAb), but not by IL-2 and IL-4 mAbs. This suggests that IL-7 is a potent regulatory cytokine capable of acting independently of IL-2 and IL-4 in the expression of CD23. Northern blot analysis showed an increase in CD23 mRNA when activated T cells were cultured in the presence of IL-7. A significant increase in receptor numbers on activated T cells was detected by Scatchard analysis when IL-7 was added to the cell cultures. The induction of CD23 expression by IL-1, IL-3, IL-4, IL-5, IL-6, interferon-8 and OKT3 on PHA-activated T cells was not of the same magnitude as observed in the presence of IL-7. These results demonstrate a selective induction of CD23 expression on activated human T cells cultured in the presence of IL-7. These data indicate that the stromal-derived growth factor points to an important role of CD23 in the regulatory network of the immune response.