The effect of selected short-chain fatty acids (SCFA) produced by anaerobic bacteria on the production in vitro of procoagulant activity (PCA) by human mononuclear cells stimulated by either Escherichia coli or Bacteroides fragilis which are common pathogens in intra-abdominal infections was investigated. In particular, acetic, propionic, succinic, butyric, and isobutyric acids were evaluated. Acetic, butyric, and isobutyric acids were able to inhibit significantly the production of PCA by bacteria-stimulated mononuclear cells. Since the production of PCA leads to the deposition of fibrin which is considered critical to the establishment of intra-abdominal abscesses, the inhibitory effect exerted by SCFA present in the inflammatory environment might play an important modulating role in the development of abscesses complicating intra-abdominal infections.