Background Macrophage migration inhibitory factor(MIF) possesses proinflammatory function when secreted from the cells, and it also exhibits antioxidant properties based on its intrinsic oxidoreductase activity.However, the role of MIF in cardiac fibrosis is not well known. In the present study, the effect of MIF on fibrosis-associated gene expression and the underlying mechanism were examined. Methods The collagen content in mouse myocardium was detected by Masson staining. Expressions of MIF and fibrosis-associated Col1a1, Col3a1 and α-SMA in mouse myocardium or mouse cardiac fibroblasts were detected by quantitative real-time PCR and Western blot assay, respectively. Mature miR-29b expressions in mouse myocardium and cardiac fibroblasts were determined by real-time PCR. Smad3 activation in MIF-treated cardiac fibroblasts was also detected by Western blot assay. Results Compared with the db / m control mice, the collagen content was significantly increased in the myocardium of diabetic db / db mice. MIF was up-regulated, but miR-29b was down-regulated in the diabetic myocardium. Quantitative real-time PCR and Western blot assay showed that MIF could inhibit fibrosis-associated Col1a1, Col3a1 and α-SMA expressions in mouse cardiac fibroblasts.Smad3 activation was inhibited, but miR-29b was up-regulated in MIF-treated cardiac fibroblasts. Enforced expression of miR-29b significantly suppressed Col1a1, Col3a1, and α-SMA mRNA and 1protein expressions in cardiac fibroblasts. Conclusions MIF possesses the anti-fibrosis activity through inhibiting Smad3activation and through up-regulating miR-29b expression, and miR-29b can inhibit fibrosis-associated Col1a1,Col3a1 and α-SMA expressions in cardiac fibroblasts. Background Macrophage migration inhibitory factor (MIF) possesses proinflammatory function when secreted from the cells, and it also exhibits antioxidant properties based on its intrinsic oxidoreductase activity. However, the role of MIF in cardiac fibrosis is not well known. In the present study, the effect of MIF on fibrosis-associated gene expression and the underlying mechanism were examined. Methods The collagen content in mouse myocardium was detected by Masson staining. Expressions of MIF and fibrosis-associated Col1a1, Col3a1 and α-SMA in mouse myocardium or mouse cardiac fibroblasts were detected by quantitative real-time PCR and Western blot assay, respectively. Mature miR-29b expressions in mouse myocardium and cardiac fibroblasts were determined by real-time PCR. Smad3 activation in MIF-treated cardiac fibroblasts was also detected by Western blot assay. Results Compared with the db / m control mice, the collagen content was significantly increased in the myocardium of diabe tic db / db mice. MIF was up-regulated, but miR-29b was down-regulated in the diabetic myocardium. Quantitative real-time PCR and Western blot analysis showed that MIF could inhibit fibrosis-associated Col1a1, Col3a1 and α-SMA expressions in mouse cardiac fibroblasts. Smad3 activation was inhibited, but miR-29b was up-regulated in MIF-treated cardiac fibroblasts. Enforced expression of miR-29b was significantly reduced for Col1a1, Col3a1, and a-SMA mRNA and 1 protein in expressions in cardiac fibroblasts. MIF possesses the anti-fibrosis activity through inhibiting Smad3 activation and through up-regulating miR-29b expression, and miR-29b can inhibit fibrosis-associated Col1a1, Col3a1 and α-SMA expressions in cardiac fibroblasts.