Lectin-stimulated supernatants of human peripheral blood leukocytes (PBL) contained an activity that maintained the long-term growth of murine interleukin-3 (IL-3) dependent cell lines such as the 32 Del line. We compared this human IL-3 activity with the basophil promoting activity (BaPA) recently described by us. Besides lectin-stimulated PBL supernatants that contained both BaPa and IL-3, we found that BaPA was produced mainly by E rosette-positive T cells or the human Mo T cell line, while IL-3 was produced by human T hybridomas and, more interestingly, by human macrophage cell lines. Based on molecular weight (BaPA: 20–30 kilodaltons, IL-3: 15–19 kilodaltons) as well as on molecular charge (BaPA: pI = 6.1–7.5, IL-3: 4.9–6.2), we concluded that human IL-3 and BaPA are distinct factors.