The effects of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3; 10 nmol/l) on the human monomyelocytic cell line U937 were investigated. Addition of 1,25-(OH)2D3 led to a decrease in cell proliferation which fell at 72 h to 67.8 +/- 4.3% (mean +/- S.E.M.) of control values. The presence of CD14, a surface marker found on mature monocytes/macrophages but not on U937 cells, was detectable as early as 18 h and peaked at 48 h, when 63.6 +/- 4.2% of the cells were positive. However, changes in c-myc mRNA levels were detected earlier, starting within 4 h of exposure to the hormone and being reduced to 38 +/- 8.2% of control values of 24 h. These effects were reversible after removal of the hormone, with the same sequence of events seen following addition of the hormone. There was first an increase in c-myc mRNA levels, starting within 2 h and reaching control values by 24 h. These changes were followed by loss of CD14 which became undetectable after 72 h. Proliferation recovered slowly and incompletely, since it was 81.7 +/- 0.7% of control after 72 h. A constant reciprocal relationship between c-myc mRNA and CD14 levels was found both in the presence and after removal of 1,25-(OH)2D3. Regulation of U937 cell proliferation and maturation by 1,25-(OH)2D3 is thus preceded by early modulation of c-myc mRNA.