We have established two rat mAbs, TUGh4 and TUGh5, specific for the human gamma chain of the IL-2 receptor (IL-2R), which is known to be shared among receptors for IL-2, IL-4 and IL-7. The antibodies bound to cell lines transfected with the human gamma chain gene but not to their parental cell lines, and precipitated 65-70 and 80-90 kDa cell surface molecules from lysates of human T cells surface-labeled with Na125I and chemically cross-linked with [125I]IL-2 respectively. Flow cytometry with TUGh4 and TUGh5 detected the gamma chain in a wide variety of peripheral blood cell populations including CD4+ T cells, CD8+ T cells, CD20+ B cells, CD56+ natural killer cells, CD14+ monocytes and granulocytes, contrasting with expression of the alpha and beta chains of IL-2R.