The c-fms proto-oncogene encodes a transmembrane glycoprotein that is closely related or identical to the receptor for the monocyte colony-stimulating factor CSF-1. The present studies examined the mechanisms responsible for the regulation of c-fms gene expression during human monocytic differentiation. Levels of c-fms mRNA were undetectable in HL-60 promyelocytic leukemia cells, while 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced monocytic differentiation of these cells was associated with the appearance of these transcripts. Run-on transcription assays demonstrated that the c-fms gene was transcriptionally active in uninduced HL-60 cells and that the rate of transcription was unchanged after TPA treatment. These findings suggested that c-fms mRNA levels in HL-60 cells are controlled by posttranscriptional mechanisms. The half-life of c-fms transcripts in TPA-induced HL-60 cells was found to be at least 6 h, while inhibition of protein synthesis with cycloheximide (CHX) decreased this half-life to 4 h. Moreover, inhibition of protein synthesis was associated with decreases in c-fms mRNA levels and a block in the induction of c-fms transcripts by TPA. These findings indicated that the c-fms transcript is stabilized by a labile protein. In contrast to HL-60 cells, c-fms mRNA is constitutively expressed in resting human monocytes and is down-regulated by treatment of these cells with TPA. Run-on assays demonstrated that TPA-induced downregulation of c-fms mRNA levels in monocytes occurred at the posttranscriptional level. Moreover, the results demonstrate that levels of c-fms mRNA are regulated posttranscriptionally by a labile protein. In this regard, the half-life of the c-fms transcript was 6.1 h in monocytes, while treatment of these cells with CHX decreased the half-life to 30 min. Furthermore, this effect of CHX occurred in the absence of changes in the rate of c-fms gene transcription. Together, these findings indicate that c-fms gene expression is regulated at a posttranscriptional level both in HL-60 cells induced to differentiate along the monocytic lineage and in human monocytes. The findings also indicate that levels of c-fms mRNA are regulated by the synthesis of a labile protein which is involved in stabilization of the c-fms transcript.