1. A method of evaluating in vitro phagocytic activity of leukocytes, using inert latex particles has been described. 2. 363 investigations have been carried out according to the method in clinically healthy donors, in patients with and without malignant disorders. The subjects have been divide into groups according to nature of the disease (operable and inoperable tumors, non-malignant, acute anc chronic disorders). 3. A statistical evaluation of the results revealed that the phagocytic capacity of leukocytes is significantly activated in the presence of a pathological process in the organism and this to a considerably higher degree in tumorous disorders than in non-malignant affections. The degree of activation of the phagocytic capacity of leukocytes can be measured by this method and the latter can be used in every haematological laboratory.