The in vitro effect of Pseudomonas aeruginosa elastase on human monocyte function was examined. Mononuclear cells isolated from the peripheral blood of healthy individuals were incubated with various concentrations of elastase, and the chemotactic activity and chemiluminescence response of these cells were measured after washing the cells free of the enzyme. Elastase at concentrations of 0.1 micrograms/ml and higher inhibited the chemotactic activity of monocytes towards F-Met-Leu-Phe and zymosan-activated serum. The inhibition of chemiluminescence response was observed at as low as 0.125 micrograms/ml elastase. Monocytes/macrophages are known to be involved in the inflammation observed in the lungs of patients infected by P. aeruginosa. The inhibition of monocyte function by elastase may serve as an escape mechanism for this bacterium from these cells.