Bactericidal, proteolytic and signal proteins released by activated neutrophils play a major role in infection fighting and inflammatory processes. These proteins are mainly stored in organelles called granules until induction of their controlled exocytosis. The present work deals with the characterization of the proteins which are secreted upon activation of human neutrophils by ionomycin and calcium. Proteins were separated by two-dimensional gel electrophoresis and identified by peptide mass fingerprinting. Almost all the previously described soluble components of neutrophil granules could be identified. Moreover, several additional proteins were shown to be secreted by activated neutrophils, namely calgranulins, human cartilage glycoprotein of 39 kDa (HC gp-39), chitotriosidase, and annexin XI. Their subcellular localization and possible functions are discussed.