Recent findings on the capacity of omeprazole to influence various leukocyte functions, in vitro, raises the question on the potential use of protonic pump inhibitors, commonly used in the treatment of acid-secretion-related disorders, as immunomodulators. The aim of this study was to evaluate the in vitro effect of lansoprazole on human natural killer (NK) cell cytotoxix activity, chemotaxis and superoxide anion (O2*-) generation exerted by polymorphonucleated cells (PMNs). NK cytotoxicity activity was assessed by a 51Cr release assay, PMN chemotaxis was determined by an under agarose method and O2*- generation was analyzed on the basis of reduced cytochrome C. Incubation times with lansoprazole was 30 min for PMNs and 1-4.5 hours for NK cells, respectively. Lansoprazole induced significant dose dependent inhibition of NK cell activity and PMN functions at concentrations ranging from 100 to 1,000 microM. This study demonstrate that lansoprazole, like omeprazole, inhibits several leukocyte functions, in vitro, then suggesting that protonic pump inhibitors are able to provoke these effects, at least at certain doses.