The biological activity of Uncaria tomentosa (Willd.) DC. (cat's claw) was evaluated by application of the chicken embryo model. Among three groups of eggs (n = 360) with twelve-day old embryos, two were injected with different doses of cat's claw extracts (0.0492 and 0.492 mg/200 lambda). To the third control group 200 lambda of physiological salt was applied. All eggs were incubated in conventional forced-air apparatus until hatched. Hatchability, chicken weight and wholesomeness were analyzed. Selected parameters of blood including number of erythrocytes (RBC), number of leukocytes (WBC), mean red cell volume (MCV), hematocrit (HCT), hemoglobin concentration (HGB), mean amount of cell hemoglobin (MCH), mean cell hemoglobin concentration (MCHC) and embryo weight (MAS) were assayed and compared. Significant differences with ANOVA were observed for MCV (P = 0.002), MCHC (P = 0.00001) and MCH (P = 0.02). Applying the chicken embryo model brought new information about the biological activity of U. tomentosa showing an unfavourable effect on some morphological blood parameters.