Using a radioimmunoassay we have determined serum levels of the carcinoma-associated antigen CA-50 in 266 patients with colorectal cancer. Elevated CA-50 levels were found in Dukes' A (15%), Dukes' B (43%), Dukes' C (31%) and Dukes' D (65%). Patients who had developed a recurrence had 66% elevated levels. 25% of resected patients with no evidence of disease also had elevated CA-50 levels. From 139 patients operated on for a Dukes' A-C, a rise in CA-50 levels from the pre- to the 6-9 month post-operative sample was demonstrated in 12 cases in the absence of any clinical evidence for a recurrence. On follow-up, a recurrence later developed in all these cases with lead times of CA-50 titre rises ranging from 5 to 40 months. A rise in CA-50 levels after resection of a Dukes' A-C is indicative of a recurrence and may precede any clinical evidence of disease by several months or years. Data is also presented from 552 cases with colorectal cancer analysed with a immunoradiometric assay.