We investigated the effect of RU41740, a glycoprotein extracted from Klebsiella pneumoniae and possessing immunomodulating properties, on human neutrophil functions in vitro and ex vivo. Our in vitro results showed that RU41740 increased complement- and Fc receptor-dependent phagocytosis. Moreover, the drug enhanced the oxidative metabolism (assessed by chemiluminescence) both in resting and stimulated cells; in the latter case the RU41740-induced enhancement was observed when neutrophils were stimulated with opsonized particles of N-formyl-methionyl-leucyl-phenylalanine (FMLP) but not when phorbol myristate acetate was used. Using otherwise effective experimental conditions, RU41740 did not affect spontaneous or FMLP-induced neutrophil migration. For the ex vivo experience we tested neutrophils of ten elderly subjects with a previously demonstrated phagocytic defect. These subjects were treated orally with RU41740 at a daily dose of 2 mg for 1 week during the first month, and of 1 mg for 1 week in the second month. In this population, RU41740 was able to restore the impaired phagocytic activity and to induce a significant increase of spontaneous chemiluminescence (CL); stimulated CL was also positively influenced. These effects on neutrophils provide new explanatory bases for the immunostimulatory activity of RU41740.