T-cell growth factor (TCGF or interleukin-2) is an inducible glycoprotein hormone of molecular weight 15,000 (ref. 1) synthesized and secreted by T lymphocytes following activation with antigen or mitogen^2,3. TCGF is required for proliferation and expansion of T cells following antigen encounter^4,5 and to maintain them in long-term culture in vitro^6–9. Full expression of the human immune response requires both the induction of TCGF synthesis and the formation of specific TCGF membrane receptors^10,11. Monoclonal antibodies binding TCGF have been prepared^12,13. In contrast, antibodies specific for the TCGF membrane receptor have not been identified, nor has the receptor been characterized. We have prepared a monoclonal antibody, termed anti-Tac^14,15, which appears to bind to the human membrane receptor for TCGF. In support of this, we now demonstrate that anti-Tac suppresses TCGF induced proliferation of T cells and blocks binding of radiolabelled TCGF to cells from a cloned human continuous T-cell line. Also we have partially purified and characterized the putative TCGF receptor. This receptor is a glycoprotein with a molecular weight (M_r) of 47,000–53,000.