A previous study reported the results of a radioimmunoassay that analysed immune complexes (IC) with a specific labeled polyclonal antibody for the detection of the early stages of colon cancer. In order to investigate further the possible clinical use of this assay, a blind study that screened 505 patients referred for colonoscopy, compared their pathology reports with the results of four assays that measure levels of circulating tumor markers. These were CEA, AFP, Ca 19-9, and our previously described radioimmunoassay (RIA). Of the patients with no malignancies, the results that were in the normal range were as follows: CEA-473/495 (95.6 per cent), Ca 19-9-486/495 (98.2 per cent) and our RIA-488/495 (98.6 per cent). AFP levels were in the normal range for all patients in the study. The only assay to identify any Dukes' C and D patients was Ca 19-9, which detected 2/3 (67 per cent). Of the patients with Dukes' A and B colon cancer, CEA only identified 1/7 (14 per cent), AFP and Ca 19-9 0/7 (0 per cent), and our own RIA 5/7 (71 per cent). The positive results of our assay were significantly different from those of the other three assays with p values all less than 0.05. These preliminary results suggest that this RIA, because of its ability to detect the early stages of colon cancer, may be an effective complement to the currently available assays. The combination may provide a more comprehensive evaluation in monitoring colon cancer.