We describe methods for the development and partial characterization of antigen-specific T-cell lines from the guinea pig, which is the classical experimental animal in tuberculosis research. T cells were obtained from strain 2 guinea pigs immunized with BCG vaccine or with killed Mycobacterium tuberculosis in oil. T-cell lines were obtained from limited dilution cloning of antigen-stimulated, blast-enriched lymphocyte cultures. The lines were grown with weekly reseedings in the alternating absence and presence of mycobacterial antigen. Antigen reactivity of the cell lines was studied with lymphocyte stimulation tests. With these methods we have consistently obtained antigen-reactive cell lines. When injected in small numbers intradermally in the presence of antigen in syngeneic guinea pigs, some of these cell lines gave rise to antigen-specific erythematous tuberculin-like skin reactions. The skin reactions, which were usually without induration, were most pronounced after 24 h. Histological examinations of skin undergoing such reactions showed that the erythemas were not accompanied by mononuclear infiltrations. We expect that transfer experiments with guinea pig T-cell lines will prove useful tools in the analysis of the contribution of defined mycobacterial antigen preparations to tuberculosis immunity.