Testicular macrophages contribute to the immune privilege of the testis through their immunoregulatory properties. The mechanisms of regulation of the essentially anti-inflammatory testicular macrophage phenotype are not well understood, but attention has been drawn to the role of the immunoregulatory cytokine, activin A. Within the immune system, activins exert both pro- and anti-inflammatory actions, depending on the immunological context. Activin A is produced by a number of testicular cell types, including the Sertoli cells, which have been shown to possess graft-protecting activities in co-transplantation studies. In order to investigate macrophage responses to activin, two murine macrophage models were compared: CSF1-matured bone marrow-derived macrophages (BMDM) and the RAW246.7 (RAW) macrophage cell line, which differ significantly in their activation status and downstream inflammatory signalling pathways. In BMDM cultures, activin A (50 ng/ml) significantly induced the secretion of the pro-inflammatory cytokine TNF upon stimulation (6 and 24h) with bacterial lipopolysaccharide (10-100 ng/ml). The secretion of the anti-inflammatory cytokine, IL-10, upon LPS challenge was significantly decreased by activin A. Additionally, the mRNA expression of pro-inflammatory markers (Tnf, Gpr18) were significantly increased by activin A in both resting and LPS-activated BMDM, while the expression levels of anti-inflammatory markers (Chil3, Mrc1) were decreased. In contrast, production of TNF was significantly decreased by activin A in RAW macrophages. A reduction in production of the pro-inflammatory mediator, NO, was observed in both macrophage types, while its anti-inflammatory counterpart, Arg1, was elevated. In summary, these studies indicate that activin A exerts predominantly pro-inflammatory effects in BMDM, whereas preliminary data in RAW macrophages suggest an anti-inflammatory response towards activin A treatment. Further investigation of the differences between the responses of macrophages under different activation states will help to elucidate the role that activin A plays in regulating macrophage activity in the testis and other tissues.