Lipid-bound sialic acids were estimated in blood serum of 118 patients with tumors, of 13 donors and of 38 patients with non-tumoral diseases. Content of the sialic acids in blood serum of the patients with growing tumors constituted 23.1 +/- 1.1 mg/100 ml, which was 2-2.5-fold higher as compared with healthy donors (10.5 +/- 0.4 mg/100 ml) or with the patients with non-tumoral diseases (13.8 +/- 0.5 mg/100 ml). In the oncologic patients during the remission period the content of lipid-bound sialic acids in blood serum approached the normal level - 12.1 +/- 0.5 mg/100 ml. The highest level of the sialic acids was found in patients with lymphogranulomatosis II-IV step, with melanoma during the period of growth, with kidney tumor and with liver tissue primary tumor. Estimation of circulating lipid-bound sialic acids in the oncologic patients may serve as a test for evaluation of the tumor growth as well as of therapeutic treatment efficiency.