We showed that the carcinoembryonic antigen (CEA) level in a peritoneal washing is an indicator of the postoperative survival of gastric cancer patients. On the premise that the polyvinylidine difluoride membrane adsorbs a fixed quantity of protein, this study was designed to produce a rapid, colorimetric, semi-quantitative assay of peritoneal CEA levels by using anti-CEA antibodies. At the time of laparotomy, peritoneal washings were collected from 60 gastric cancer patients, and CEA levels were determined by our assay and by an enzyme immunoassay (EIA) method. The accuracy of our method corresponded with the results of the EIA method. All the cases with high levels of CEA in the peritoneal washings showed positive color changes. Our new assay had no relation to protein concentrations of the samples. The assay makes use of diluted peritoneal washings without adaptation by protein concentration. The measurement can be completed in the operating room within 30 min. As a result, this assay can detect peritoneal microdissemination easily during surgery, and it can be used as an indication of intraoperative chemotherapy against peritoneal micrometastasis.