Colony-stimulating factors (CSF) are important factors in the proliferation and differentiation of hematopoietic progenitor cells (HPC), and in the survival and activation of mature blood cells. Interleukin-1 (IL-1) combined with fetal bovine serum (FBS) strongly induces the expression of macrophage-CSF (M-CSF), granulocyte-CSF (G- CSF), and granulocyte-macrophage-CSF (GM-CSF) in fibroblasts. Here, we report on the regulation of CSF gene expression in murine fibroblasts following IL-1 and FBS stimulation. We demonstrate that 10T1/2 murine fibroblasts induced by FBS or IL-1 accumulate M-CSF messenger RNA (mRNA). G-CSF mRNA expression was induced by IL-1, and not by FBS. For GM-CSF expression, induction with both FBS and IL-1 was required. Blocking studies with actinomycin-D showed that active transcription is essential for accumulation of all three CSF mRNAs. After blocking protein synthesis with cycloheximide, IL-1- or FBS-induced M-CSF expression and IL-1 plus FBS-induced GM-CSF expression still occurred and was increased. IL-1-induced G-CSF expression was completely prevented in these cells by pretreatment with cycloheximide, illustrating that, for this effect, intermediate protein synthesis was required. The half-lives of M-CSF transcripts were not substantially altered by addition of IL-1, FBS, or FBS plus IL-1. Using nuclear run- on assays, we demonstrated that the transcription rate of M-CSF was increased up to 20-fold by the addition of FBS, IL-1, or FBS plus IL-1. After blocking protein synthesis with cycloheximide, IL-1-or FBS- induced increase in M-CSF transcription rate was also observed. GM-CSF transcription increased up to fourfold after induction with FBS or IL- 1. G-CSF transcription rate was not altered by FBS or IL-1. Our results indicate that M-CSF expression induced by FBS or IL-1 in these fibroblasts is primarily regulated at the transcriptional level. GM-CSF expression appears to be regulated both transcriptionally and posttranscriptionally, and G-CSF expression is regulated mainly at the posttranscriptional level.