Thymus-derived lymphocytes exert a number of regulatory and effector activities in the immune system. Evidence has accumulated in recent years that these activities are largely executed by distinct subsets of T cells that can be distinguished by particular cell surface antigens (1, 2). This has led to the concept of a complex network of T cell types, each endowed with its own specific function. Rigorous proof of this concept has been difficult to achieve with heterogeneous cell populations. Recently, it has been possible to establish and maintain for prolonged periods in culture antigen-specific T cell lines (3, 4), paving the way to a more definitive approach to this question. Homogeneous populations of antigen-specific T cells should also prove extremely useful for studies of the chemical nature of the antigen in its activating form, as well as the nature of the T cell antigen receptor itself. In addition, comparison of these parameters between different functional subsets of T cells specific for the same antigenic determinant should be possible.We report here the establishment, genetic control, and preliminary characterization of the antigen specificity of normal T cell lines specifically responsive to a structurally defined epitope, L-tyrosine-p-azobenzenearsonate (ABA-Tyr)) It had been established previously (5, 6) that this simple synthetic compound is immunogenic in guinea pigs and mice. Accordingly, A/J mice were immunized with ABA-Tyr, and lymph node cells from the immune animals were used to establish antigen-reactive T cell lines. Strain A/J mice were chosen for this purpose because their anti-ABA antibody response is dominated by a major cross-reactive idiotype (7), providing the possibility of useful markers for the purification and characterization of T cell antigen-specific molecules as well as studies of the regulation of idiotype expression in antibody responses. The functional activity of these ABA-Tyr-specific T cell lines will be the subject of another communication.