Humoral autoimmunity of patients with malignant gastrointestinal tumors was studied by different variants of enzyme immunoassay. In the preoperative period the frequency of detecting autoantibodies binding to DNA and components of external and internal membranes of cells of different tissues was not higher in patients (n-55) than in donors (n-45). Beginning from the 7th postoperative day all patients were found to have a significantly increased level of autoantibodies to DNA (p less than 0.01), fibroblasts (p less than 0.01), hepatocytes (p less than 0.01), islet cells (p less than 0.01), hepatic or pancreatic microsomes (p less than 0.05). There was a tendency towards normalization of these level by the 21st day in all cases. In patients (n-10) treated by blood intravascular laser irradiation after the operation the level of IgG (but not IgM) autoantibodies to DNA by the 14th postoperative day was significantly (p less than 0.01) higher than that in patients whose blood was not exposed to radiation. It is suggested that the discovered humoral immune reaction is a necessary component of wound immunity and develops in response to hetero-organic autoantigens which are unmasked in injury of the tissues during the operation. Laser therapy ensures a more intensive development of this reaction without altering its direction.