The effect of rhamnolipid purified from culture supernatant of Pseudomonas aeruginosa on human peripheral blood neutrophil and monocyte function was studied. It was shown that rhamnolipid at concentrations of up to 100 micrograms/ml did not affect neutrophil and monocyte chemotaxis towards various chemoattractants. The rhamnolipid by itself did not show chemotactic activity and did not induce any oxidative burst response. Preincubation of monocytes with rhamnolipid enhanced the oxidative burst response of these cells to phorbol myristate acetate (PMA) and to opsonized zymosan by 2 to 5 fold. The priming effect was observed both in a superoxide assay and in a chemiluminescence assay. However, rhamnolipid did not prime the neutrophil oxidative burst response. Monocytes/macrophages are involved in the inflammatory process in the lung infections caused by P. aeruginosa and oxygen radicals are known to cause tissue damage. Therefore the priming by rhamnolipid of monocytes for enhanced generation of oxygen radicals may play a role in the pathogenesis of tissue damage in the lungs of cystic fibrosis patients with P. aeruginosa infections.