This chapter discusses the antigen-specific suppressor factors from hybridoma cell lines. Given the extreme heterogeneity of T cell subsets and the number of antigenic specificities that each subset can recognize, T cell receptors and antigen-specific regulatory molecules are difficult to isolate and characterize. One method for obtaining sufficient numbers of homogeneous populations that are representative of rare cells is the stomatic cell hybridization technique. The two most important requirements for the production of T cell hybridomas are a reproducible, rapid assay system and a well-characterized T cell activity. T cell hybridomas can be constructed which constitutively produce GAT-TsF or GT-TsF that are functionally and serologically identical to factors extracted from or secreted by normal T cells. In addition, monoclonal GAT-TsF and GT-TsF induce specific unresponsiveness by the activation of Ts2 cells. T cell hybridomas are also produced by fusion of Ts2 cells and BW5147. © 1985, Elsevier Inc. All rights reserved.