Macrophage-mediated activation of helper T cells results in the production of a series of antigen-nonspecific factors which have dramatic immunoregulatory effects on target macrophages, B cells and T cells. We have utilized phorbol myristic acetate-stimulated EL-4 thymoma cells to study various aspects of the production and mechanism-of-action of one of these factors Interleukin 2 (IL 2). Despite the fact that PMA induces the production of IL 2 in the EL-4 cells, the general rate of protein, RNA, and DNA synthesis of the cells is drmatically decreased. We have purified the EL-4-derived IL 2 to approximately 4,000,000 units of IL 2 per mg of protein and in the process shown the factor to be distinct from both colony-stimulating factor and B cell growth factor. Studies on the interaction between IL 2 and its receptor indicate that receptor-mediated endocytosis of IL 2 by specific receptors on suppressor T cells may represent one mechanism by which T cells can non-specifically suppress immune responses.