Transcription of early growth response gene-1 (Egr-1) is required for macrophage differentiation. Since granulocyte-macrophage colony-stimulating factor (GM-CSF) induces macrophage proliferation as well as the transcription of Egr-1 mRNA, we characterized the induction of Egr-1 protein in murine peritoneal macrophages following treatment with GM-CSF. As determined by Western blot analysis, the concentration of GM-CSF required to induce detectable Egr-1 protein in macrophages was 50 units/ml, and the inducible Egr-1 protein species was 80 kDa. Following stimulation of macrophages with GM-CSF, Egr-1 protein was detected within 25 min and reached maximum level at 70 min. The concentration of GM-CSF that was required to induce Egr-1 protein in macrophages was similar to that required to induce macrophage proliferation. A similar concentration has been detected previously in mouse serum exposed to bacterial endotoxin in vivo, suggesting that the 80 kDa Egr-1 protein may be induced in macrophages under the physiologic circumstances of pathogen invasion. Thus, our data support further study on the role of Egr-1 protein in mediating GM-CSF induction of macrophage proliferation.