Interleukin-5 (IL-5) has previously been isolated only as a product of T lymphocytes. We have found that Epstein-Barr virus transformed B lymphocytes produce large amounts of IL-5 activity in culture supernatants, inducing proliferation of murine BCL1 cells, and supporting the selective growth of eosinophil colonies in semi-solid culture. Production of IL-5 messenger RNA by transformed B-cell lines was verified by Northern analysis using a 3.2-kilobase cloned DNA fragment containing the full-length human IL-5 gene, and immunoreactive IL-5 was detected in B-cell culture supernatants. These findings suggest a possible expanded role for the B cell in the induction of eosinophilia, and should serve as a focus for additional investigation into possible roles for IL-5 in human B-cell proliferation and differentiation.