The origin of myeloperoxidase (MPO) in bronchoalveolar lavage (BAL) was investigated in the first part of the study. Radioimmunoassay of the cellular and supernatant MPO content as well as the peroxidase-antiperoxidase (PAP) technique were employed to determine the cellular source of MPO. The concentrations of MPO were measured in serum and BAL in the second part of the study. The aim was to determine whether the capillary bed was also a source of MPO. Neutrophil numbers in BALs obtained from 20 healthy subjects correlated significantly to the concentrations of MPO in cell-free BAL supernatants (r = 0.643, P less than or equal to 0.01). The cellular content of MPO in mixed BAL cells was significantly correlated to the number of neutrophils in the mixture (r = 0.536, P less than 0.05), but not to the number of any other cells. Moreover, the PAP-technique identified MPO in lung tissue neutrophils in resection specimens obtained from three patients undergoing surgery. This technique also revealed strong MPO activity in all BAL neutrophils and a weak activity in merely 4% of the alveolar macrophages in cytospin preparations obtained from seven BALs. High BAL/serum ratio of MPO concentrations suggests that MPO is of local origin, rather than passively diffused from the circulating pool. We therefore conclude that strong evidence suggests that MPO in BAL originates from lung neutrophils and that BAL MPO content may be used to estimate the neutrophil presence or activation in epithelium lining fluid.