For the purpose to obtain the information of the mechanism of protein uptake by the tumor cells, some cytochemical and electron microscopic observations were carried out on Ehrlich ascites tumor cells incubated with
horseradish peroxidase (basic hemoprotein, molecular weight approximately 40,000) in vitro. In the earlier periods of the incubation peroxidase was found to be adsorbed on some area of surface of the tumor cells forming a thin protein layer, where an active pseudopodia formation was observed. With the lapse of time, the protein was taken in the deep cytoplasm by the infoldings
of the cell membrane and accumulated in the cytoplasmic vesicles having limiting membrane. Concerning the accumulation of the protein into the vesicles, small tubular structures in the cytoplasm connecting the cell
surface and the vesicles, were considered to participate in the intracellular transportation of peroxidase taken up. In cold environment (2°C), the formation of pseudopodia and deep inward infoldings of the cell membrane
was inhibited and simultaneously the uptake of peroxidase stopped. Iodoacetate and sodium fluoride also effected to suppress the pseudopodia and infoldings formation moderetely, as well as uptake of peroxidase, though they did not stop completely. These facts have indicated that
horseradish peroxdase is taken up by Ehrlich ascites tuimor cells through pinocytosis which involves energy-requiring process dependent upon glycolytic metabolism of the tumor cells.</p
