The differentiation into macrophages of the monocytic cell line U-937 or the monomyelocytic cell line HL-60 induced by phorbol esters is accompanied by a rapid induction of the expression of the proto-oncogene c-fos. Maximal levels of c-fos gene transcripts accumulate within 20-30 min following induction. By 2 hr, the c-fos-specific mRNA level drops 5 to 10 fold below that detected at 30 min after induction. The concentration of c-fos mRNA remains unchanged for the next 5 days, at which time over 99% of the viable precursor cells are converted to adherent macrophages. c-fos protein can be detected within 20 min of induction, reaching maximal levels by 40-60 min. Little or no c-fos protein is detected by 120-150 min after induction, even though c-fos mRNA remains detectable. Presence of cycloheximide leads to superinduction of c-fos mRNA transcripts. No c-fos gene transcripts are detected when the HL-60 cell line is induced with Me2SO to differentiate to granulocytes. A role for the c-fos protein is envisaged in the monocyte differentiation pathway.