Surgical human thymus, upper respiratory tract, lung and small and large bowel specimens were analyzed for the presence of interleukin 2 receptor (IL2-R)-positive cells. Histochemical (toluidine) and immunologic (anti-IL2-R monoclonal antibody) staining procedures revealed a distinct anti-IL2-R positivity in most of metachromatically staining cells. These positive cells were observed not only in tissues showing strong inflammatory reaction and mast cell hyperplasia, as in Crohn's disease, but also in those not histologically affected by pathologic conditions. This finding suggested that human mast cells, like T blast cells, express the p55 chain of IL2-R on their surface. To see whether IL2-R was being actively synthesized, a cell preparation rich in peripheral blood basophils (PBB), which are cells closely related to mast cells, was obtained. Ultrastructural analysis of PBB after indirect immunogold procedure revealed that the vast majority expressed the IL2-R. Moreover, the presence of intracellular reaction products in the cytoplasm of most membrane-positive PBB was indicative of active antigen synthesis. Furthermore, Northern blot analysis evidenced specific IL2-R mRNA in PBB, while its expression was augmented several times when PBB were cultured in the presence of stimulated T cell supernatant.