Concentrations of extracellular deoxy- and ribonucleic acids in blood plasma and cell-surface-bound of blood cells were investigated in healthy donors and patients with malignant gastrointestinal tumors. Our results indicate that high concentration of extracellular DNA in blood plasma along with decreased level of extracellular RNA on the surface of blood cells correlate with development of gastrointestinal cancer. Ratio of nucleic acids in plasma to total amount of nucleic acids circulated in blood is a characteristic parameter distinguishing cancer patients from healthy persons.