Results of studies demonstrating the identification of the interleukin-2 receptor (IL-2R; i.e., anti-Tac) on the membrane ultra-structure of human leukemic T cells with an antibody carrying an electron dense colloidal gold microsphere (e.g., immunogold) that was visualized using a scanning electron microscope (SEM) are reported. Our IL-2R model system employed HTLV-1 retrovirus-infected lymphoblastoid cells of the long-term human leukemic T cell line HUT-102B2. The presence of the IL-2R on these cells was defined using a double antibody procedure that employed as the primary antibody a purified mouse monoclonal anti-Leu-IL-2R antibody (mIgGlk, anti-Tac, CD25), and used as the secondary antibody a goat anti-mouse IgG (gamma-chain specific) antibody that had been covalently bonded to a 40 nm colloidal gold particle. More than 95% of the HUT-102B2 were IL-2R+, and there was a uniform distribution of the IL-2R over the surface of the cells. Corresponding controls were employed in all examinations and included IL-2R- Jurkat human leukemic T cells and isotype identical immunoglobulins. The primary and secondary antibody reagents contained whole human serum and bovine serum albumin, and there was no evidence of the non-specific binding of these antibodies. These studies are the first to demonstrate the presence of a lymphokine receptor on the surface architecture of a cell. We anticipate no difficulty in applying the immunogold/SEM technology to define both normal and malignant cell membrane receptors for other cytokines.