Five hybridoma T cell lines were prepared by fusion of Ts3 cells with the BW 5147 thymoma. The culture supernatants from these T cell hybrids contained a factor, TsF3, which specifically suppressed 4-hydroxy-3-nitrophenyl acetyl hapten (NP(-hapten cutaneous sensitivity responses. The properties of this new series of hybridoma factors was compared with those of two previously characterized types of NP-specific suppressor factors (TsF1 and TsF2). TsF3 activity was only observed if the factor was administered during the effector phases of the immune response. TsF3 bears I-J and C57BL anti-NP antibody idiotypic determinants and has binding specificity for the NP hapten. Furthermore, TsF3 does not suppress H-2 (I-J)-incompatible mice. In addition to this H-2 restriction, the monoclonal TsF3 factors also demonstrated an Igh genetic restriction. Finally, the TsF3 factors could be distinguished by their ability to suppress cyclophosphamide-treated recipients.