Recently we described a murine T-cell hybrid that produces activities that promote the differentiation of eosinophils (eosinophil differentiation factor) and cause proliferation of the BCL1 B-cell lymphoma (B-cell growth factor II activity). Both activities appear to be associated with the same molecule, which has therefore been termed interleukin 4. The hybrid does not produce any other known lymphokines. We now find that purified interleukin 4 has no effects on small resting B cells but induces naturally occurring large B cells (which have presumably been preactivated in vivo) to synthesize DNA and to secrete IgM and low levels of IgG. B cells activated by anti-Ig antibodies apparently only become responsive to the factor once they have reached late G1 stage. All bioactivities of interleukin 4 are associated with a protein of Mr 44,000 (by NaDodSO4/PAGE). Therefore these results demonstrate that this lymphokine alone is sufficient to induce clonal expansion and maturation of activated B cells.