A comparative analysis of the secondary structure of blood serum proteins was carried out in patients with multiple myeloma, chronic lymphocytic leukemia and in healthy donors. The secondary structure of proteins in serum was determined using an infrared spectroscopy method. It was demonstrated that the proposed approach allows one to register a decrease in the portion of α-helical regions and an increase in the number of β-sheets in the blood serum samples of patients with multiple myeloma compared with healthy donors and patients with chronic lymphocytic leukemia.