Small, resting B lymphocytes express few, if any, interleukin 2 (IL-2) receptors, but activated B cells may express such receptors. This paper examines the requirements for receptor expression. Normal murine splenocyte populations were enriched for B cells and cultured at relatively low density. IL-2 receptor expression was studied by measuring the binding of the anti-IL-2 receptor monoclonal antibody PC61. Lymphoblasts arising through stimulation by Escherichia coli lipopolysaccharide failed to express IL-2 receptors. B cells cultured with conditioned medium from concanavalin A-stimulated EL4 thymoma cells, with or without LPS, displayed IL-2 receptors. This bioactivity of EL4 conditioned medium could not be replaced by any concentration of B-cell-stimulatory factor 1 (IL-4), IL-1, IL-2, or IL-3 tested. However, the recently cloned lymphokine T-cell-replacing factor (IL-5) was a potent inducer of IL-2 receptor expression, as was the probably identical material known as eosinophil differentiation factor. The receptors so induced appeared to be functional, as receptor-expressing (but not control) lymphoblasts, responded to IL-2 by proliferation, indicative of high-affinity-receptor expression.