We examined the effect of interleukin-3 (IL-3) on human CD4+ cloned T cells, P607 and 1C2. By flow cytometric analysis, we found that IL-3 downregulated the surface expression of IL-2 receptor (R) beta chain in a dose-dependent manner but had little effect on that of IL-2R alpha chain. A simultaneous 125I-labeled IL-2 binding assay showed a decrease in the number of high-affinity, but not of low-affinity, IL-2Rs by IL-3. The downregulation of the IL-2R beta chain began 3 hours after culture initiation, increased further thereafter, and was completely inhibited by anti-IL-3 antibodies. Expression of mRNA for either alpha or beta chain was not reduced by IL-3, and this suggests that the reduction of surface beta chain expression was not caused by the reduction of beta chain mRNA. IL-3-accelerating internalization of IL-2R beta chain appeared to be one of the mechanisms for IL-3-induced downregulation of surface IL-2R beta chain expression. IL-3 alone increased the proliferation of T-cell clones but decreased the existing increment of their proliferation by IL-2. Accordingly, IL-3 may be one of the factors acting as a liaison between the hematopoietic and immune systems.