Suppressor T cell hybridomas specific for L-glutamic acid 60-L-alanine30-L-tyrosine10 (GAT) elaborate monoclonal suppressor factors (TsF) and bear surface markers such as H-2, I-J, and Thy-1 antigens. Over a period of years in culture, the production of TsF by such hybridomas has been constitutive and continuous; by contrast, the expression of surface markers has declined and some have been lost. The mixture of lymphokines in the supernatant fluid from concanavalin A-activated mixtures of allogeneic splenocytes (CAS) restored surface antigen expression, and the effective agents have been identified by using purified sources of lymphokines. Murine gamma interferon (IFN(γ) induced expression of H-2 class I antigens, but not I-J or Thy-1, in the hybridoma cells. Human interleukin 2 (IL-2) induced the increased expression of I-J and Thy-1 but not H-2 class I antigens. Interestingly, production of TsF was decreased by IFN(γ) treatment but increased by IL-2, indicating some measure of regulation by lymphokines of the level of a protein that is constitutively produced.