Background: B cell-activating factor belonging to the TNF family (BAFF) is primarily expressed by macrophages and dendritic cells, and stimulates B cell proliferation, differentiation, survival, and Ig production. In the present study, we explored the effect of activin A on BAFF expression by APCs. Methods: To investigate the effect of activin A on BAFF expression by mouse APCs, we measured the level of BAFF expression at the transcriptional and protein levels using RT-PCR and ELISA. Results: Activin A markedly enhanced BAFF expression in mouse macrophages and dendritic cells at both the transcriptional and protein levels. SB431542, an activin receptor-like kinase 4 (ALK4) inhibitor, completely abrogated activin A-induced BAFF transcription. Furthermore, overexpression of DN-Smad3 abolished activin-induced BAFF expression at the transcriptional and protein levels. Conclusion: These results demonstrate that activin A can enhance BAFF expression through ALK4-Smad3 pathway.