The activation of T lymphocytes for antigen-induced proliferative responses (1), T lymphocyte cytotoxicity (2), delayed-type hypersensitivity (3), and helper function for antibody responses (4) requires their recognition of the nominal antigen on the surface of an antigen-presenting cell (APC) 1 bearing Ia-DR molecules encoded by the I region of the major histocompatibility locus. In addition to the requirement for "joint recognition," the APC provides a second signal for T lymphocyte activation, the cytokine interleukin 1 (IL-1) (5). Monocytes and other cell types, including dendritic cells, all have been reported to produce IL-1 and to be effective APC (6-8). Recently, normal B cells (9), B cell tumors expressing Ia antigens (10-12), and Epstein-Barr virus (EBV)transformed B cell lines (13) all have been reported to exert an effective antigenpresenting function for T lymphocyte activation. Furthermore, murine B cell lymphoma cells have been shown to "process" and present antigens (I0) in a manner similar to that reported for murine (14) as well as human monocytes (15). These B cell lines, however, have not been reported to secrete IL-1 or any other soluble factors, raising critical questions concerning the importance of such second signals for T cell activation. In this study we report that ROHA-9, a human EBV-transformed lymphoblastoid cell line that is an effective APC, constitutively produces a soluble factor with biochemical and biological characteristics resembling those of monocytederived human IL-I. The IL-ll ike activity was undetectable in crude supernatants, but was evident after gel fractionation studies which, in addition, revealed the presence of a coexistent inhibitory factor. The biochemical and biological properties of this inhibitory factor have also been investigated.