T-cell hybridomas were first produced some 20 yr ago (1 ). They have been used for research in almost all aspects of T-cell biology ranging from experiments on T-cell receptors and their specificity to the cytokines produced by T cells and the factors that lead to T-cell death. For biological studies, T-cell hybridomas have several advantages over normal T cells and T-cell lines. T-cell hybridomas grow rapidly in tissue culture. Their proliferation does not require components other than those of normal medium and serum. The cells divide spontaneously. Normal resting T cells do not divide in culture and must be freshly isolated from animals immediately before use. T-cell lines and clones do divide in vitro, however they require stimulation in the form of antigen and antigen presenting cells and/or cytokines such as interleukin 2 (IL-2). These nontransformed cells are therefore more trouble and more expensive to culture than hybridomas are and there is always the danger that inappropriate cells from the antigen presenting population will contaminate the specific T-cell preparation. There is also the danger that T-cell lines will senesce, a phenomenon which does not apply to hybridomas. T-cell hybridomas are usually easy to clone and may have plating efficiencies approaching the ideal. Finally, because they grow so well in culture, T-cell hybridomas, unlike their untransformed counterparts, can be selected for mutants, infected and transformed, thus allowing genetic manipulations that are not possible in normal T cells.