When polymorphonuclear leukocytes (PMNL) and soluble or particulate matter interact, the cells produce chemiluminescence. The chemotactic peptide formylmethionyl-leucyl-phenylalanine induced a two-peak chemiluminescence response in PMNL. The response was modified, both in magnitude and in the time course of the response, when PMNL were incubated at 22 degrees C for 15 to 120 min before the addition of stimulus. The cellular response to formylmethionyl-leucyl-phenylalanine was also changed by the addition of cell-free supernatants from stored PMNL suspensions to freshly prepared PMNL before the addition of formylmethionyl-leucyl-phenylalanine. This indicated that PMNL during storage released substances that changed the chemiluminescence response. A similar change could be obtained by addition of purified myeloperoxidase to freshly prepared PMNL before the addition of stimulus, indicating that the myeloperoxidase-hydrogen peroxide system can function as a modulator of the cellular response to chemotactic factors.