The results presented in this paper demonstrate that the chemotactic peptide N-formylmethionylleucylphenylalanine (f-Met-Leu-Phe) is rapidly inactivated by the products of the respiration of human neutrophils stimulated by the peptide itself. The process of inactivation is impeded by the addition of inhibitors of myeloperoxidase (KCN, NaN3), of catalase, of methionine but not by the addition of superoxide dismutase, indicating that the mechanism of inactivation is the oxidation of methionine residue by myeloperoxidase-H2O2-halide system. The oxidation of the peptide causes the rapid cessation of the respiratory burst, since the sulfoxide derivative loses its ability to bind the specific receptors of neutrophil surface and, hence, its biological activity. The comparison between the time course of the binding of f-Met-Leu-[3H]Phe to the specific receptors and the rate of the respiratory response of neutrophils in the presence and in the absence of the process of peptide oxidation was used to investigate the mechanism of the activation of the respiratory burst by the peptide-receptor complexes. In conditions where the inactivation of the stimulatory agent takes place the stimulated respiration slows down and resumes the resting state shortly after the cessation of the binding, although a substantial amount of the peptide remains bound to the specific receptors. In conditions where the degradation of the peptide does not occur the binding of the peptide and the respiratory burst continue for a longer period of time, but the rate of the respiration, calculated in terms of the instantaneous velocity (Vist), is not correlated to the amount of the ligand bound to the membrane receptors measured at various times, indicating that a summation of the effects of the ligand-receptor complexes does not occur as they form. These findings demonstrate, as far as the respiratory response is concerned, that the biological activity of the peptide-receptor complexes is short-lived and that continuous de-novo receptor occupancy is necessary for the maintenance of the activated respiration.