Extracts of cotton dust were tested for their ability to activate the alternative complement pathway in fresh normal human serum (NHS). Alternative pathway activation was determined by a haemolytic assay utilizing glutathione-sensitized human erythrocytes, consumption of alternative pathway components in terms of alternative pathway CH50 units and an immunoelectrophoretic assay to detect split products of activation of factor B. All assays were performed under conditions that have been shown to block the initial steps of classical pathway activation but permit activation of the alternative complement pathway. Results demonstrate that the cotton dust extracts could consume alternative complement pathway proteins in a dose-response manner. The complement activating factor is probably endotoxin since a cotton dust extract obtained by an extraction method for endotoxin yielded the greatest activity.