Activin A, a member of the transforming growth factor-beta superfamily, is a critical early mediator of acute inflammation. Activin A release coincides with the release of tumour necrosis factor-alpha (TNF-alpha) in models of lipopolysaccharide (LPS)-induced inflammation. The source of circulating activin A during acute inflammation has not been identified and the potential contribution of leukocyte subsets was examined in the following study. Human leukocytes from healthy volunteers were fractionated using Ficoll gradients and cultured under serum-free conditions. Freshly isolated human neutrophils contained 20-fold more activin A than blood mononuclear cells as measured by enzyme-linked immunosorbent assay (ELISA), and both dimeric and monomeric forms of activin A were detected in these cells by western blotting. Activin A was predominantly immunolocalized in the neutrophil cytoplasm. Purified neutrophils secreted activin A in culture when stimulated by TNF-alpha, but were unable to respond to LPS directly. Although TNF-alpha stimulated activin A release from neutrophils within 1 h, activin subunit mRNA expression did not increase until 12 h of culture, and the amount of activin A released following TNF-alpha stimulation did not change between 1 and 12 h. Specific inhibition of the p38 MAP kinase signalling pathway blocked TNF-alpha-induced activin release, and the secretion of activin A was not due to TNF-alpha-induced neutrophil apoptosis. These data provide the first evidence that neutrophils are a significant source of mature, stored activin A. Stimulation of the release of neutrophil activin A by TNF-alpha may contribute to the early peak in circulating activin A levels during acute inflammation.