We have previously reported that the human monocytoid cell line U-937-1 constitutively expresses transforming growth factor-alpha (TGF-α) and that the steady-state levels of TGF-α mRNA as well as TGF-α protein release increase when U-937-1 cells are differentiated towards monocytes/macrophages. Interleukin-6 (IL-6), which has been shown to have growth-stimulatory effects on a number of cell types, has recently been shown to enhance TGF-α expression in keratinocytes. In the present study we investigated whether TGF-α expression in macrophage-like cells could be regulated by IL-6 using U-937-1 cells as a model system of monocyte/macrophage differentiation. U-937-1 cells were differentiated with retinoic acid (RA), vitamin D3 (Vit-D3) or phorbol-12-myristate-13-acetate (PMA) for 4 days and were then treated with human recombinant IL-6 (1000 IU/ml) for up to 24 hr. Northern blot analysis revealed that cells differentiated with PMA, inducing the phenotype of a secretory macrophage, markedly increased their TGF-α mRNA levels (2.7-fold) when treated with IL-6; the response was maximal at 6 hr and remained high at 12 hr. The expression of the TGF-α gene was accompanied by release of TGF-α protein into the cell culture medium, irrespective of differentiating agent, as demonstrated by enzyme-linked immunosorbent assay (ELISA), as well as by surface expression of pro-TGF-α as determined by indirect immunofluorescent cytometry. However, the superinduction of the TGF-α gene by IL-6 in cells differentiated with PMA was not accompanied by any increase in TGF-α protein release or pro-TGF-α surface expression. We conclude that since IL-6 causes increased steady-state levels of TGF-α mRNA in macrophage-like cells, it may prime these cells for production of this growth factor. Furthermore, we have shown that the IL-6 receptor complex is functional in U-937-1 cells induced to differentiate towards a secretory macrophage by treatment with PMA.