Human neutrophils were found to actively migrate through 5µ pores of filter membranes towards aqueous extracts of cotton dust. An extract of cotton dust that had previously undergone a repeated base and heat treatment was virtually inactivated in this model. When budesonide, an anti-inflammatory steroid, was incubated with the neutrophils at a concentration of 50µg/ml., it was found to significantly inhibit the chemotactic response to cotton dust extract. However, a single 5mg. dose of the steroid given to guinea-pigs by intra-peritoneal injection, prior to inhalation of cotton dust extract or bacterial lipopolysaccharide, failed to significantly reduce the airway recruitment of neutrophils in-vivo following examination by broncho-alveolar lavage, The results suggest that budesonide is effective at reducing the amoeboid activity of neutrophils. The large influx of neutrophils into the airways, which is a feature of the response to cotton dust, is likely to involve mechanisms in addition to the direct chemotactic activity of the dust alone.