The value of radioimmunoguided surgery in first and second look laparotomy for colorectal cancer. Radioimmunoguided surgery (RIGS) using an anti-CEA (A5B7) monoclonal antibody has been assessed in 52 patients (43 primary excisions and nine second look procedures) undergoing surgery for colorectal carcinoma. The antibody localized in 97.8 percent of primary tumours and in 88.8 percent of the principal tumor in second look procedures. Additional information concerning the extent of primary tumor was obtained in 11 of 43 patients (25.5 percent) undergoing excision of primary carcinoma and five of nine patients (55 percent) in the second look series. Incorrect information was obtained about the extent of the primary tumour in six patients (11.3 percent), whereas no incorrect information was obtained during second look procedures. RIGS correctly predicted the subsequent Dukes' staging in 77 percent of first look cases (sensitivity 65 percent, specificity 90 percent), although accurate identification of individual nodes was impossible. The technique influenced the surgical procedure performed in 2 of 43 cases (4.6 percent) in primary surgery and in three of nine patients undergoing second look laparotomy (33 percent). RIGS in primary colorectal carcinoma may provide additional information concerning extent of locally advanced tumors in particular and the principle that the subsequent surgery may be influenced has been established. The technique appears to have a greater role in second look procedures where it may help determine the extent of recurrent tumour. Larger follow-up series are required to define how the additional information provided by this technique may best be exploited. 