Compared to TGF-β1 treatment alone (5 ng/ml), both test concentrations of C3 exotoxin significantly (p < 0.05) abrogated cyclin D1 protein expression in both normal and IPF lung fibroblasts, irrespective of subsequent TGF-β1 exposure. This evidence concerns the gene TGFB1 and idiopathic pulmonary fibrosis.