Investigations on 279 patients with colon carcinoma revealed an increase in the sensitivity of the CEA (carcinoembryonic antigen) test with regard to tumor stage from 42% (Dukes A) to 86% (Dukes D). Curative operable carcinoma without lymph-node or distant metastases had CEA levels up to 70 ng/ml. CEA values below 20 ng/ml had no predictive value concerning resectability at first operation. When the CEA level rose above 20 ng/ml, the proportion of curative operations fell from 83% (CEA values below 20 ng/ml) to 17%. Recurrence rate rose with the preoperative level of CEA from 22% (CEA less than 2.5 ng/ml) to 62% (CEA greater than 10 ng/ml). The resection rate at second operation had a closer relationship than at first operation to the preoperative CEA level; it fell from 50% (CEA less than 5.0 ng/ml) to 0% (CEA greater than 20 ng/ml). At operation, highly differentiated tumors were found to be at an early stage in 50% of cases, compared to 26% of G2 and 11% of G3 tumors. In 44% of dedifferentiated tumors CEA levels were above 20 ng/ml. An influence of tumor differentiation on CEA remained even after division into the individual tumor stages.