It was shown previously that inhalation of tannin caused the recruitment of neutrophils and macrophages into the lungs. This study was designed to examine whether tannin also influenced the functional capacity of these inflammatory cells to produce hydrogen peroxide. Conditioned rabbit alveolar macrophages and human neutrophils were used. The results with macrophages showed that low tannin concentrations increased the production of peroxide, while higher concentrations inhibited peroxide production. Con A stimulated macrophages lost their ability to produce peroxide if pretreated with tannin in a dose dependent manner. In neutrophils, tannin slightly enhanced peroxide release at all concentrations used. It also increased PMA stimulated peroxide production. These results demonstrated that tannin has distinctly different effects on hydrogen peroxide production in neutrophils and macrophages. This difference might be of importance in the acute inflammatory reaction in byssinosis.