By adding a drop of the serum of dogs with ancylostomiasis in the medium of bone marrow tissue culture of normal dogs, the author has examined its influences on the growth rate, white blood cell function (wandering velocity, phagocytosis of carbon particles and vital staining of neutral red) in hanging drop culture and increasing rate of the red blood cell count and hemoglobin content in fluid culture. 1) As for the growth rate, it was slightly increased in the later atage of tissue culture. 2) Function of the neutrophils was not impedented on wandering velocity, phagocytosis of carbon particles and vital staining of neutral red, but rather slightly accerelated in the early stage of tissue culture. 3) According to the above mentioned results, the author has presumed that some stimulative substances for the bone marrow activity and neutrophils function are contained in the serum of hook wormdog. 4) Wandering velocity of the eosinophils was slightly increased, so that the author has resumed that it had occured from the stimulation of hook worm toxin possessing positive taxis for the eosinophils. 5) Increasing rate of the red blood cell count was not suffered from by adding the serum of hook worm dog on fluid culture, but that of hemoglobin content was slightly declined, which did not come from the restrain of hook worm toxin for the formation of hemoglobin, but from iron deficiency in the serum.