In a primary immune response, B cells require signals from the T-cell lymphokines interleukins 2 and 5 (IL-2 and IL-5) to develop into IgM-secreting cells. One role of IL-2 and IL-5 is to activate transcription of the gene encoding the IgM joining component, the J chain. In this study the activation mechanism was investigated by using an inducible beta-lymphoma cell line to examine J-chain RNA expression and factor binding to the J-chain promoter. The analyses revealed that both IL-2 and IL-5 trigger a decrease in the binding of two promoter-specific nuclear proteins that precedes the appearance of J-chain RNA. In combination the two lymphokines effected nearly additive changes in factor binding and J-chain RNA abundance. Both effects were reversed upon withdrawal of the lymphokine stimulus and both were inhibited in the presence of the T-cell lymphokine IL-4. These findings indicate that the IL-2 and IL-5 signal pathways converge to deliver a common signal that regulates the repressor activities of two lymphokine-responsive promoter elements.