One of the limitations of intraoperative tumor detection with radiolabeled monoclonal antibody (Mab), by means of a gamma-detecting probe (GDP), is the long time interval needed between Mab injection and surgery to obtain low blood-pool activity. Such an interval can be shortened considerably, exploiting the high affinity between avidin and biotin. METHODS Twenty patients with colorectal cancer were injected with 1 mg of biotinylated 125I monoclonal antibodies followed, 48 hr later, by a chase of cold avidin. During surgery, the GDP was used to detect radioactive emissions from the tumor and normal tissue. Tumor tissue samples were analyzed in vitro by immunohistochemical tests for the presence of tumor antigens and in vivo antibody localization. RESULTS At the time of surgery (average 7 days postinjection), the mean value of circulating radioactivity was 6% +/- 3% of the injected dose. Of 20 patients studied, tumors were localized in 13 cases (65%). Subclinical tumors were detected in 3 patients (15%). CONCLUSIONS The use of 125I-labeled biotinylated Mabs followed by avidin as a chase enhances the applicability and effectiveness of radioimmunoguided surgery technology and will allow the use of radioisotopes with a shorter half-life than 125I.