Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pleiotropic hematopoietic growth factor that induces both growth and differentiation of tissue macrophages. The subcellular mechanism of action of GM-CSF is unknown. We have examined the effect of GM-CSF on the immediate early response gene, Egr-1, in murine peritoneal macrophages. Our data demonstrate that recombinant GM-CSF (25 units/ml) produces a 12-fold increase in Egr-1 mRNA within 30 min. Pretreatment with cycloheximide (10 micrograms/ml) had no effect on the ability of GM-CSF to increase Egr-1 mRNA. In nuclear runoff studies, GM-CSF increased the transcription rate of Egr-1 by 10-fold at 10 min. The maximal effect on Egr-1 transcription occurred at 25 min (13-fold) and decreased by 45 min. The half-life of Egr-1 mRNA in GM-CSF-treated macrophages is 13-21 min. We were unable to calculate the half-life in control cells, however, because of the short half-life and low level of constitutive expression of Egr-1 mRNA. Endogenous protein kinase C activity in macrophages was depleted by treatment with 12-O-tetradecanoylphorbol-13-acetate for 24 h. GM-CSF increased Egr-1 mRNA in protein kinase C-depleted macrophages, whereas the stimulatory effect of 12-O-tetradecanoylphorbol-13-acetate on Egr-1 was blocked. These data show that GM-CSF rapidly increases transcription of Egr-1 mRNA. The effect of GM-CSF on Egr-1 mRNA does not require de novo protein synthesis or protein kinase C. These findings provide a basis for investigating the molecular mechanism of action of GM-CSF in tissue macrophages.