T lymphocytes represent an extraordinarily heterogeneous population of cells with respect to antigen receptors, differentiation antigens, and distinct immunologic functions. Because of this heterogeneity, it is of obvious advantage in many studies to use restricted and homogeneous T cell populations. Recently, two approaches to isolate homogeneous clones of T lymphocytes have become feasible. One of them takes advantage of the observation that T lymphocytes can be maintained and propagated in vitro by culture in media containing the T cell growth factor interleukin 2 (1). Although this technique allows for the growth of clones of T cells with distinct helper, suppressor, or cytotoxic properties (2-4), the clones remain absolutely dependent on interleukin 2, and large numbers of cells are difficult to generate. The second approach uses techniques similar in principle to those defined by Kohler and Milsteiu (5) for the production of monoclonal antibodies. This technology has been developed in murine systems where a number of investigators have created T cell hybrids by fusing normal murine T cells with continuously growing, aminopterin-sensitive murine T lymphoma cell lines. Some of these hybrids express phenotypic properties of both parental ceils and, in many instances, they have exhibited functional properties of the mature fusion partners (6-13). In the experiments to be reported below, we have extended this latter technology to human T cells. First, we established 6-thioguanine (TG)l-resistant, aminopterinsensitive mutants from the continuously growing human T cell line CEM. One of these mutant T cell lines (CEM-T15) was fused with lectin-stimulated human T cells and, after selection in medium containing hypoxanthine, aminopterin, and thymidine (HAT), T cell hybrids were obtained. These human T cell hybrids grow independently from any added T cell growth factor(s). Some of these hybrids express the O K T 3 antigen that is found on normal T cells but not on immature thymocytes or on the CEM-T15 mutant line (14). Functional analysis demonstrated that many of these human T cell hybids exhibit helper function in vitro. Several of these helper T cell hybrids have been cloned repeatedly and maintain helper function.