A new group of gastric cancer associated antigens in sera of patients with rectal cancer, benign diseases and normal human were detected by MG series monoclonal antibodies against gastric cancer. The average values of normal persons plus 3 standard deviations was set as the highest limit of normal value. The positive rate were 69.7% (23/33) in sera of patients with rectal cancer, and 4%(1/25) in sera of patients with benign diseases. The values of MG-Ags were decreased to normal level 8-10 days after surgical removal in 5 patients of rectal cancer. The results of test suggest that determination of MG-Ags in serum of patients might be helpful in the diagnosis of rectal carcinoma.