The influence of taurine on neutrophil phagocytic and bactericidal capacities and lysosomal enzyme-releasing ability was evaluated in the present study using neutrophils obtained from casein-elicited rat peritoneal exudates. Taurine was dissolved in drinking water at a concentration of 0.3%, and the solution was given to rats for 1-21 days (460 mg/kg/day). Taurine concentration in the serum increased with the term of its administration, while in the neutrophils, it increased significantly after administration for 1 or 3 days. When administered for 7 or 10 days, however, no difference was noted from the control group, but then the concentration remarkably increased after 21 days of administration. The bactericidal capacity of the neutrophils against Escherichia coli was strengthened as their concentration of taurine increased; phagocytic capacity was also strengthened. The release of myeloperoxidase following phagocytosis of yeasts increased with administration, while the release of beta-glucuronidase, lysozyme and lactate dehydrogenase, which are induced by N-formylmethionyl-leucyl-phenylalanine, were inhibited. The hypotonic hemolysis of erythrocytes was also inhibited. Taurine decreased the fluorescence depolarization of diphenylhexatriene, indicating an increase in membrane fluidity. These results suggested that taurine strengthens both phagocytic and bactericidal capacities of neutrophils by increasing the fluidity of neutrophil membrane and membrane stability and thus plays an important role in the mechanism of host defense.