AbstractIn man, B cell maturation factors obtained from T cells or T cell lines have been shown to induce antibody formation in mitogen or anti‐immunoglobulin activated B cells, and in some continuous B cell lines, but the relationships between these factors and B cell differentiation factors in antigen‐specific antibody responses is unclear. We have now shown that supernatants from phytohemagglutinin‐activated tonsil cells, or from the Gibbon Ape T cell line ML A‐144, can substitute for T cells in the specific antibody response by human blood B cells to influenza virus. Thus, T cell‐depleted non‐rosette‐forming (E−) cells prepared from peripheral blood mononuclear cells made antibody when cultured with antigen and factor together, whereas control cultures of E−cells with either antigen or factor alone did not. Moreover, E−cells cultured with factor and influenza virus strain A/X31 made antibody to A/X31, but not the non‐cross‐reacting strain, B/HK (andvice versa)showing that the response was antigen specific. The activity in these supernatants, therefore, fulfilled the functional definition of T cell‐replacing factor (TRF). The possibility that interleukin 2 (IL2) present in the TRF‐containing supernatants was expanding residual T cells in the E−preparations to provide normal T cell help was excluded in three different ways. First, E−cells depleted of T (Leu4+) cells to undetectable levels made normal amounts of antibody when cultured with antigen and TRF. Secondly, a limiting dilution technique was employed to show that help in cultures of E−cells and TRF was not mediated by antigen‐specific T helper cells. Thirdly, TRF‐containing supernatants depleted of IL2 retained activity, whereas purified IL2 was inactive. Preliminary purification of TRF by gel filtration on Ultrogel AcA54 columns showed that all the activity eluted in a single peak between 35000 and 43000 molecular weight. This distinguishes human TRF from IL2 and from other B cell maturation factors with a molecular weight range of 15000–20000 which act on continuous Bcell lines.In addition to TRF, supernatants from phytohemagglutinin‐activated tonsils also contained a factor which could induce polyspecific IgM production, but only in cultures containing significant numbers of T cells. This additional activity may have been due to IL2, and provides an explanation for the apparent T cell‐dependent effects sometimes observed in experiments designed to test B cell differentiation