Examinations of the protein composition of erythrocyte membrane in patients with cancer of the stomach, large intestine and rectum by using the method of disc-electrophoresis in polyacrylamide gel revealed the changing contents of protein fractions, which consisted in a lower share of high-molecular polypeptides with, simultaneously, higher quantity of low-molecular peptides. A comparison of the studied parameters of the protein spectrum of erythrocyte membrane as observed in patients with cancer of the stomach, on the one hand, and in patients with tumors of the large intestine and rectum, on the other hand, led to defining the reliable difference only for the content of the fraction-5 proteins.