The effects of IL-6 on plasmacytomas and on normal B and T cells were examined. Evidence was presented to indicate that, in the mouse, IL-6 not only supports the growth of plasmacytomas in vitro, but also that it significantly enhances tumor progression in vivo. Analysis of the response of normal mouse B cells to IL-6 revealed the existence of a unique synergy between IL-6 and IL-1. The results obtained with T cells also highlighted the remarkable effects of the IL-1-IL-6 combination. In several experimental systems, including the generation of allogeneic cytolytic T cell responses, it appeared that accessory cells could be completely replaced by IL-1 and IL-6, whereas either cytokine used by itself was completely inactive. Analysis of the mode of action of IL-6 in T cell activation demonstrated the existence of two distinct mechanisms: induction of IL-2 biosynthesis and enhancement of T cell responsiveness to IL-2 and IL-4. Finally, it was reported that IL-6 is essential only during the early phases of T cell activation, thus indicating as far as T cells are concerned that IL-6 must be considered as a competence factor.