The antibody response to infectious microbes is regulated by a series of interaction among T cells, B cells, and macrophages. Each B cell is genetically determined to maturate antibody (immunoglobulin, Ig)-producing cells against a distinct antigenic determinant on an infectious microbe, and the antibody produced plays a key role in the Immoral immune response against invading microorganisms. The B cell response to an antigen is regulated by a helper T cell responding to, and specific for, the same antigen molecule. Helper T cells recognize antigenic peptides in context of class II major histocompatibility complex (MHC) molecules on B cells, and secrete several soluble factors which can induce growth and maturation of B cells (23,28,36). IL-5 was initially described as a factor that induces terminal differentiation of already activated murine B cells into Ig-secreting cells and originally designated as T-cell-replacing factor (TRF) (56,65).The establishment of a TRF-producing T-cell hybrid B151K12 (B151) which does not secrete detectable levels of other lymphokines affecting B cells demonstrated that TRF is a lymphokine distinct from IL-1, IL-2, IL-3, IL-4, IL-6, and interferon-gamma (IFN-r) (63, 64). TRF produced by B151 was purified to homogeneity and shown to promote DNA synthesis of murine chronic B cell leukemic cells, BCL1 or of dextran sulfate-stimulated normal B cells, and this activity of TRF is called B-cell growth factor (BCGF) II activity (17, 58).We prepared rat anti-mouse TRF mAb (TB13 and NC17) by using HPLCpurified B151-TRF as irnmunogens (18). Both TB13 and NC17 mAbs blocked biological activity of B151-TRF on B cells and BCL1 cells, whereas they did not inhibit biological activities of other cytokines. B151-TRF was purified by using TB 13-coupled immunoaffinity column and found to have an approximate molecular mass of 46 kDa under nonreducing conditions and of 23 to 26 kDa under reducing conditions on SDS-PAGE analysis, indicating that B151-TRF comprises a dimer form.The molecular cloning of complementary DNA (cDNA) encoding murine TRF has convincingly demonstrated that a single molecule is responsible for both TRF and BCGFII activities (27,42). Although TRF was initially believed to be principally active on B cells, recombinant TRF/BCGFII has been shown to be active on T cells and eosinophil progenitors as well as mature eosinophils (42,66, 77). We therefore proposed that TRF/BCGFII be called 59 3