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W orking with cultured dermal fibroblasts derived from control individuals and patients with systemic sclerosis (SSc), we have examined the effects of protein kinase C-δ (PKC-δ) on type I collagen biosynthesis and steady-state levels of COL1A1 and COL3A1 mRNAs. Rottlerin, a specific inhibitor of PKC-δ, exerted a powerful, dose-dependent inhibition of type I and type III collagen gene expression in normal and SSc cells. Optimal rottlerin concentrations caused a 70–90% inhibition of type I collagen production, a >80% reduction in COL1A1 mRNA, and a >70% reduction in COL3A1 mRNA in both cell types. In vitro nuclear transcription assays and transient transfections with COL1A1 promoter deletion constructs demonstrated that rottlerin profoundly reduced COL1A1 transcription and that this effect required a 129-bp promoter region encompassing nucleotides –804 to –675. This COL1A1 segment imparted rottlerin sensitivity to a heterologous promoter. Cotransfections of COL1A1 promoter constructs with a dominant-negative PKC-δ expression plasmid showed that suppression of this kinase silenced COL1A1 promoter activity. The results indicate that PKC-δ participates in the upregulation of collagen gene transcription in SSc and suggest that treatment with PKC-δ inhibitors could suppress fibrosis in this disease.