Several laboratories have recently confirmed the importance that interleukin 2 (IL-2, formerly referred to as T cell growth factor) plays not only in the maintenance of cultured functional T cell lines but also as a potent regulator of immune responses. Despite the development of preparative IL-2 purification procedures, as well as identification of IL-2 producer tumor cell lines, simple sources of lectin-free IL-2 containing conditioned medium remain undefined. In hopes of establishing such a source of mitogen-free IL-2, we elected to fuse the high titer IL-2 producer tumor cell line, LBRM-33, with a drugmarked T cell lymphoma. In this communication we report the successful isolation of a T cell hybridoma that constitutively synthesizes and secretes biologically active IL-2. Hybridoma conditioned medium contained approximately 10 to 100 times the amount of IL-2 produced by conventional T cell mitogen-stimulated murine splenocyte preparations. In addition to providing a simple means for producing IL-2 containing conditioned medium without the presence of contaminating lectin, we hope that constitutive IL-2 producer hybrid cell lines will prove interesting as vehicles for dissection of the molecular events involved in IL-2 biosynthesis.