We report a prospective study in two groups of colorectal cancer patients carried out by radio-immunolocalization (RIL) with F(ab') fragments of monoclonal antibodies against CEA and CA 19.9 labeled with 131-I. Twenty-two patients were studied before radical surgery and 12 patients after initial surgery, when progressive increase in CEA was registered. Scintigraphic images obtained in vivo in RIL studies were compared with scintigraphic images of the corresponding surgical specimens. Results were compared with known serum marker levels and with the presence and localization of markers in the excised specimens. RIL images correctly identified 13 of 23 (52%) primary tumors, with only one false positive image. Scintigraphy of surgical specimens correlated with RIL findings in 14 of 19 cases (74%). Four specimens which showed antibody uptake had not been visualized preoperatively in the RIL study. Two of them were retrovesical and were obscured by residual activity in the bladder. Nine of 13 (64%) patients with at least one elevated tumor marker were imaged. Staining pattern or intensity of antigen staining in the specimens did not correlate with RIL findings. Recurrent disease was confirmed by laparotomy or other exploration in 10 of the 12 patients with progressive CEA elevation during follow-up. Spontaneous normalization of CEA levels was observed in the remaining 2 patients. RIL studies were positive in 7 of the 10 patients with confirmed recurrent disease. Of the 3 false negative patients 2 had liver metastases and one developed clinical lung, bone and adrenal metastases 11 months later. No false positive studies were observed in this group.(ABSTRACT TRUNCATED AT 250 WORDS)