Abstract The activity of RU 41740, a glycoprotein extract from Klebsiella pneumoniae, endowed with immuno-modulating properties, has been investigated on polymorphonuclear (PMN) leukocyte functions. This report deals with the effect of RU 41740 on oxidative metabolism (assessed by chemiluminescence, O 2 consumption and O 2 − production) and on chemotaxis and random migration (using agarose and Boyden chamber techniques). PMNs were collected from the rat pleural cavity after induction of one acute inflammatory reaction (pleurisy induced by injection of a suspension of calcium pyrophosphate crystals). Experiments were performed in parallel after in vivo treatment or incubation in vitro. RU 41740 enhanced PMN oxidative metabolism and inhibited PMN chemotaxis while random migration was only affected using agarose assay at high concentration. This effect on PMN migration was observed with at least two attractants. These observations have been obtained either after incubation in vitro or administration in vivo. The minimal effective dose was 1 mg/kg in vivo and 0.1 μg/ml in vitro. These data suggest that RU 41740 acts directly on PMN membrane receptors.