Phagocytosis of Staphylococcus epidermidis by human neutrophils is accompanied by HCN liberation. The amount of HCN evolved is significantly higher when the bacteria are damaged by penicillin. One of the substrates yielding HCN during phagocytosis are N-dichloroglycyl residues of bacterial peptidoglycan formed by the chlorinating myeloperoxidase-H2O2-Cl- system of neutrophils. HCN deriving from the bacterial structures constitutes 6-12% of total HCN liberated during phagocytosis, which indicates that there are other substrates for HCN production. Chlorination process within phagocytosing neutrophils seems to be essential for formation of both, total HCN and that deriving from bacterial structures.