The influence of water extract of Uncaria tomentosa (Willd.) DC. bark on Pseudomonas aeruginosa infection was studied. The preparation was obtained by extraction of bark Uncaria tomentosa with water (37°C, 24 h) and further fractionation. Inbred C57BL/6 mice 7-9 weeks old, ca 20 γ of body mass, females, were fed water extract for 7 days (10, 20 or 100 mg/kg), or water (controls). On the 8th day some mice were infected i.p. with P. aeruginosa strain ATCC. Livers were excised after four hours, homogenized and the numbers of viable bacteria were estimated by plating. After administration of Uncaria water extract a significantly decreased number of bacteria in P. aeruginosa infected mice livers, as compared with the control group, was demonstrated. The inhibitory effect of dose 10 and 20 mg/kg was highly statistically significant but highest dose of extract (100 mg/kg) has not evoked important changes in the number of bacteria.