S. aureus serine proteinase inactivates human alpha-1-proteinase inhibitor (alpha-1-PI) by attacking a single peptide bond between Glu354 and Ala355 giving a modified inhibitor which is a tight complex of Mr = 4,000 and 48,000 fragments. In the present paper we show that this proteolytically inactivated alpha-1-PI is a potent chemotactic factor for human neutrophiles at a nanomolar concentration, and we discuss its potential involvement in the inflammatory reaction due to S. aureus infections.