Analysis of surface fluorescence with flow cytometry has revealed the presence of membrane-associated interleukin 2 (IL-2) epitopes on the surface of long term human T cell clones. These IL-2 epitopes could not be accounted for by soluble IL-2 binding to its specific receptor or adsorbing nonspecifically to the cells. The level of surface IL-2 antigenic determinants on the T cell clones was decreased in the presence of phorbol esters and increased in the absence of an exogenous source of IL-2. It was completely lost upon stimulation of the clones to produce the soluble lymphokine. Surface IL-2 epitopes were also detected on the Jurkat tumor cell line which secretes IL-2 upon stimulation and on another T cell tumor line MOLT 4. MLA-144 produces IL-2 constitutively; however, it did not possess membrane-associated epitopes. Tumor lines of other lineages were negative. A subpopulation of peripheral blood T lymphocytes demonstrated some membrane-bound IL-2, whereas non-T peripheral blood mononuclear cells were negative. Thus, cells with the potential of producing and secreting IL-2 upon stimulation possessed the surface epitopes of the lymphokine and cells either actively secreting IL-2 or without the potential for secretion were negative for surface expression. Membrane-associated IL-2 antigenic determinants appear to represent a T lymphocytic surface marker of potential cellular function. The relationship of this marker to the secreted lymphokine is not known. Although it is possible that the epitopes seen were present on a distinct molecule independent of secreted IL-2, the distribution on a variety of T cells and regulation via cellular activation suggest that the surface expression of IL-2 epitopes is in some way related to the soluble lymphokine.