Lipase was isolated from P. aeruginosa by ultrafiltration of sterile-filtered culture supernatant. Gel filtration on Sepharose 4B yielded a broad peak corresponding to a molecular mass range of 100 to 1000 kDa. Electron microscopy of a negatively stained lipase preparation after Sepharose 4B chromatography revealed spherical particles with diameters ranging from 5 to 20 nm. Biochemical characterization and SDS polyacrylamide gel electrophoresis suggested that these particles consisted of protein and carbohydrate including lipopolysaccharide with the major enzyme activity being lipase. Various concentrations of this lipase preparation were preincubated with human peripheral blood neutrophils and monocytes. The chemotaxis and chemiluminescence of these cells were then determined. It was shown that lipase inhibited the monocyte chemotaxis and chemiluminescence, whereas it had no or very little effect on neutrophils. The inhibitory effect was concentration dependent and was abolished by heat treatment of the enzyme at 100 degrees C. Since monocytes are one of the important cells of the host defence system the inhibition of the function of these cells may contribute to the pathogenesis of infections caused by P. aeruginosa.