Immune complexes (IC) were measured in 66 gastric cancer patients, using the 3.5 percent polyethylene glycol (PEG) precipitation method. Preoperatively, the IC values in patients with advanced gastric cancer (stages III and IV) were significantly higher than in normal subjects (p less than 0.01). In the presence of 3.5 percent PEG-precipitated IC from sera of gastric cancer patients, the mitogen responses of normal peripheral blood lymphocytes was inhibited; the PHA response revealed a significant negative correlation with the concentration of IC (p less than 0.01). Our data suggest that IC may be a major serum factor exerting immunosuppressive effect in cancer hosts.