Lymphokine requirements for the in vitro proliferation of the spleen-dependent B cell lymphoma BCL1 have been analysed. Cells were found to respond by proliferation to added recombinant (r) interleukin-4 (IL-4), r-IL-5 and recombinant granulocyte-macrophage colony-stimulating factor (r-GM-CSF). Inhibition by antibodies specific for each of these lymphokines has confirmed growth factor-dependent growth. Anti-GM-CSF has, however, been found to inhibit the proliferation of BCL1 cells induced by r-IL-4 and r-IL-5, as well as r-GM-CSF, suggesting that BCL1 cells may express receptors for GM-CSF and that GM-CSF may be able to act synergistically with IL-4 and IL-5 in promoting cell proliferation. Anti-IL-6 antibody was also found to be a very effective inhibitor of BCL1 proliferation induced by either IL-4 or IL-5 but not by GM-CSF. Added IL-6 did not stimulate BCL1 proliferation, suggesting that endogenous IL-6 may regulate the autocrine growth of BCL1 cells. BCL1 cell proliferation in vitro appears to be regulated by interactions between multiple growth factors.