Effects of human recombinant interleukin-4 (IL-4) on cord blood cells depleted of T cells and monocytes were tested in colony assays and liquid cultures. IL-4 did not induce colony formation in semisolid medium, but enhanced generation of basophil colonies induced by conditioned medium (CM) of the bladder carcinoma cell line 5637. In liquid cultures, variable degrees of basophil growth were observed in the presence of IL-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, and 5637 CM, or even with IL-4 alone, but the highest number of basophils were obtained when IL-4 was used in combination with IL-3 or 5637 CM. Progressive basophil growth was observed during 3 to 4 weeks of culturing, whereafter the numbers of basophils remained stationary for another 3 weeks. Interestingly, cord blood cell cultures performed with IL-3 contained variable percentages of eosinophils that were further enhanced in the presence of combinations of IL-3 and IL-4. These latter cultures contained approximately 50% eosinophils and 50% basophils. Kinetic studies indicated that basophils were present 7 days after onset of the cultures, whereas eosinophils did not appear before day 13. In contrast to the pronounced effects of IL-4 and 5637 CM on basophil development, relatively low numbers of eosinophils were observed under these culture conditions. Our results indicate that eosinophil and basophil development are regulated by different sets of factors, and that IL-4 has an enhancing effect of both cell lineages in association with the appropriate factors.