Following activationin vitro, peripheral blood mononuclear cells (PBMC) express cell-associated interleukin-2 receptors (IL-2R) and also release soluble IL-2R into culture supernatants. The present studies were undertaken to define which normal cells were responsible for the release of soluble IL-2Rin vitro. Both cell-associated and soluble IL-2R were quantitatively measured with a “sandwich” enzyme-linked immunoassay employing two monoclonal antibodies. PBMC were separated into populations of surface immunoglobulin-negative cells (T cells and monocytes) and surface immunoglobulin-positive cells (B cells and monocytes), and the T-cell population was further separated into OKT4-positive (OKT4+) cells and OKT4-negative (OKT4−) cells. Following activation with phytohemagglutinin, pokeweed mitogen, and the monoclonal antibody OKT3, large amounts of soluble IL-2R were released by PBMC, unseparated T cells, OKT4+ T cells, and OKT4− T cells. The population containing B cells and monocytes made small but readily detectable amounts of soluble IL-2R when stimulated with these T-cell mitogens; likely the result of contaminating T cells in the population. However, when highly purified B cells were stimulated withStaphylococcus aureus Cowan and recombinant IL-2, they also released small amounts of soluble IL-2R. The release of soluble IL-2R by T cells appeared monocyte dependent when OKT3, but not phytohemagglutinin, was employed for activation, and monocytes themselves released no detectable IL-2R under the conditions employed. These studies define the cellular requirements for the release of soluble IL-2Rin vitro and demonstrate that such receptors are released by B cells, T cells, and both OKT4+ and OKT4− T-cell subsets.