We have previously demonstrated that human promyelocytic HL-60 cells express transforming growth factor-alpha (TGF-alpha) during granulocytic differentiation. The present experiments were carried out in order to determine whether cells differentiated towards monocytes/macrophages will analogously express the TGF-alpha proto-oncogene product. HL-60 cells were induced to differentiate with 1 microM 1,alpha 25-dihydroxycholecalciferol (vitamin D3), and the human monocytoid cell line, U-937, was induced with 1 microM retinoic acid (RA), 0.1 microM vitamin D3, or 0.16 microM phorbol-12-myristate-13-acetate (PMA), ie experimental protocols known to induce monocyte/macrophage differentiation in these cells. In HL-60 cells, lacking constitutive TGF-alpha mRNA, vitamin D3 caused expression of the TGF-alpha gene and protein as demonstrated by Northern blot analysis and enzyme-linked immunoabsorbant assay (ELISA). In U-937 cells, showing constitutive TGF-alpha expression, RA but not vitamin D3 or PMA, caused marked increase in TGF-alpha mRNA (approximately 5-fold) and protein (approximately 3-fold) levels. In both cell lines the increase in TGF-alpha mRNA was evident within 24 h and continued throughout the observation period. Thus, it is established that differentiation of human leukemia cells towards monocytes/macrophages may be accompanied by TGF-alpha gene and protein expression in vitro. This is in conformity with the observed ability of mature activated macrophages to produce TGF-alpha.