Five hybridoma T cell lines were prepared by fusion of second order suppressor T cells (Ts2) with the BW5147 thymoma. The culture supernates from these T cell hybrids contained a factor, TsF2, which specifically suppressed 4-hydroxy-3-nitrophenyl acetyl hapten (NP)-induced cutaneous sensitivity responses. TsF2 activity was observed when the factor was administered during the effector phases of the immune response. TsF2 bears I-J determinants and has binding specificity for NPb idiotypic determinants. TsF2 suppressor activity could be absorbed on antigen-primed H-2-incompatible T cells but cannot suppress H-2-incompatible mice. In addition to this H-2 restriction, which maps to the I-J subregion, monoclonal TsF2 also has an Igh genetic restriction. The present results are combined with previous data to describe the cellular interactions leading to immune suppression.