Interleukin-5 (IL-5) and IL-6 have both been reported to act as B-cell differentiation factors by stimulating activated B cells to secrete antibody. However, it has not been possible to directly compare the effects of these two lymphokines because of the lack of a suitable B-cell line capable of responding to both. We have identified a clonal, inducible B-cell lymphoma, CH12, that has this property. Both IL-5 and IL-6 can independently stimulate increases in steady-state levels of immunoglobulin and J-chain mRNA and proteins, and they both induce the differentiation of CH12 into high-rate antibody-secreting cells. Nevertheless, there are significant differences in the activities of these two lymphokines. First, while IL-6 acts only as a differentiation factor, IL-5 also augments the proliferation of CH12 cells. Second, the differentiation stimulated by IL-5 but not by is partially inhibited by IL4. Inhibition of IL-5-induced differentiation was not at the level of IL-5 receptor expression, since IL-4 did not inhibit IL-5-induced proliferation. Third, IL-5 but not IL-6 stimulated increased mouse mammary tumor proviral gene expression in CH12 cells. These results demonstrate that while both IL-5 and IL-6 may act as differentiation factors for B cells, they induce differentiation by using at least partially distinct molecular pathways. Our results also establish that B cells characteristic of a single stage of development can independently respond to Lymphokines are important in regulating different aspects of the activation, growth, and differentiation of B lymphocytes. Several lymphokines, including interleukin-5 (IL-5) and IL-6, appear to be involved in inducing murine B cells to secrete antibody. IL-5 was originally described as a factor that induced both growth and differentiation of B cells (reviewed in references 37 and 38). IL-5 enhances the growth of B cells stimulated with dextran sulfate, but not with anti-immunoglobulin (anti-Ig) and IL4, and induces growth and differentiation of certain B-cell lymphomas, including BCL1 (28,34,35,37) and CH12 (21,31,36). IL-6 was originally described as a human B-cell differentiation factor that acts on activated, but not resting, B cells and certain B-cell leukemias (reviewed in references 17 and 41). Unlike IL-5, IL-6 fails to stimulate B-cell growth (2, 18, 26), although it does act as a potent growth factor for certain myelomas and hybridomas (40,42).The relative requirements for IL-5 versus IL-6 as a B-cell differentiation factor are an enigma. In the mouse, IL-5 has been shown to be the essential lymphokine required for the differentiation of antibody-secreting B cells under certain experimental conditions (5,32). These data contrast with studies of humans, in which IL-6 has reproducibly been shown to be the necessary and sufficient differentiation factor for B cells (17,18,26). However, recent results from murine studies suggest that IL-5 and IL-6 may be required under distinct experimental conditions and that IL-5 may be the critical lymphokine only when B cel...