When antigen activates B cells with the help of T cells, factors are produced by T cells which induce proliferation and maturation to Ig-secreting cells. T-cell lines and T-cell hybridomas have been obtained which, upon stimulation by antigen or by concanavalin A, produce these B-cell replication and maturation factors. However, their ability to produce these but not other factors, such as the T-cell growth factor, appears to be unstable even upon repeated recloning of the T hybridoma cells. Mitogens are known to replace some of the signals required in T-cell-dependent, antigen-specific activation of B cells. Depletion of cells, however, abolishes the mitogen responsiveness of the B-cell population from spleen. This responsiveness can be repaired when accessory cells such as peritoneal cells, irradiated spleen cells, cells of the macrophage line P388D1 or those from macrophage colonies grown from bone marrow cells with colony-stimulating factor are added back. Soluble factors obtained from different activated macrophages as well as from activated T cells also restore responsiveness. These results argue against a single, non-specific signal model of mitogenic activation of B cells and indicate that this activation is T-cell- but not macrophage-independent.