Uncaria tomentosa (una de gato, cat’s claw, vilcacora) is a liana belonging to the Rubiaceae family. For hundreds years it has been used by Indians in treating various diseases including cancers. In the present paper we evaluate the in vivo influence of Uncaria tomentosa bark water and ethanol extract on the metabolic activity of blood granulocytes in mice. Mice were fed for 7 days the both of extracts in daily dose 200 µg. The metabolic activity of granulocytes was determined by the measurement of their chemiluminescent activity in scintillation counter, after stimulation by Zymosan. Present results showed that water extract highly stimulated the granulocyte chemiluminescence but ethanol extract slightly decreased the O2-production and number of leukocytes.