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C3H/HeN mice were immunized to induce contrasuppressor T-cell (Tcs) activity, splenic T cells from these mice were fused with the BW5147 thymoma, and the resulting hybridomas were tested for their ability to produce a contrasuppressor T-cell factor (TcsF). Nine TcsF-producing hybridomas were preliminarily identified by their ability to inhibit the effect of antigen-specific suppressor T-cell factor (TsF) on the adoptive transfer of contact sensitivity. One of these hybrids, AF5.C6, was cloned, the production of a contrasuppressor factor confirmed, and the high-titred TcsF produced by this cloned hybrid characterized. Hybridoma-derived TcsF is antigen-specific and specifically binds its antigen, but does not bear immunoglobulin (Ig) determinants. Thus, hybridoma-derived TcsF is serologically and functionally identical to an antigen-specific contrasuppressor factor for contact sensitivity, whose production from splenocyte cell cultures has previously been described. The generation of a hybridoma secreting a contrasuppressor factor identical to that produced by spleen cells significantly strengthens the hypothesis that the phenomenon of T-cell contrasuppression is mediated by a specific subset of cells whose activity is contrasuppressive. The further advantages of employing T-cell hybridomas for functional, biochemical and molecular genetic analyses of contrasuppression are also discussed. [ABSTRACT FROM AUTHOR]