Both tumour necrosis factor α (TNF-α) and phorbol 12-myristate 13-acetate (PMA) stimulated human cytomegalovirus (HCMV) major immediate early (IE) enhancer/promoter activity in the HL-60 granulocyte/monocyte progenitor cell line when added to transfected cells. In U-937 monocytic cells, by contrast, TNF-α had no stimulatory effect and the addition of PMA produced only marginal stimulation. In the mature THP-1 monocytic cell line and in differentiated HL-60 cells, addition of TNF-α caused inhibition of the IE enhancer/promoter activity. The stimulating effect of PMA, as observed in the other cell lines, however, remained. Thus the effect of TNF-α on the major IE enhancer/promoter activity is determined by the degree of differentiation of the infected cells. Unlike TNF-α and PMA, the interleukins IL-1, IL-3, IL-6 as well as the cytokine GM-CSF were found to have no detectable influence on the activity of the IE enhancer/promoter activity which, likewise, was not affected by the presence of the modulator sequence. Since premonocytic cells are suggested to be sites of HCMV latency, the stimulation by TNF-α could be of potential pathophysiological significance.