A new group of gastric cancer associated antigens (MG-Ag) in ascitic fluid and pleural effusion was detected. First of all, the monoclonal antibodies (MG series) were purified and coupled to the miniglobules. The samples to be examined were then mixed with the MG-miniglobules to react. After being blocked by serum of normal mouse, the MG-miniglobules were mixed and made to react with the monoclonal antibodies labeled with HRP. With ELISA method, the MG-Ag levels in ascitic and pleural fluid were determined in 171 patients. The mean value of 87 non-malignant patients plus 3 standard deviations was arbitrarily set as the highest normal limit. Values above this limit were found in 39 (75%) of 52 patients with lung cancer and 20 (83%) of 24 patients with gastric cancer. The MG-Ag level was elevated in 4 (4.6%) of 87 patients with benign diseases. It is not elevated in 8 patients with ovarian cancer. It is suggested that determination of MG-Ag in ascitic and pleural fluid can be used for ascertaining the nature of ascites and pleural effusion.