Radioimmunoscintigraphy (RIS) and radioimmunoguided surgery (RIGS) were assessed for their usefulness in patients with colorectal carcinoma. Twenty-nine patients (18 primary tumours, 10 with a suspicion of recurrence and one colonic diverticulitis) were studied. Radioimmunoscintigraphy was performed 48 and 72 h after the injection of an anti-TAG72 monoclonal antibody (CYT-103) labelled with 111In. Radioimmunoguided surgery was performed between 72 and 96 h post-injection. During surgery, a systematic screening was performed with a hand-held gamma detecting probe and a surgical index (tumour-to-normal tissue) was obtained. There were statistically significant differences between counts in normal tissue versus tumour (P < 0.001) and RIGS was considered positive for the detection of tumour if the ratio between the counts in the area suspicious of tumour and the counts in the normal tissue was greater than 1.5. The overall sensitivity for RIS and RIGS was 71.4% (55.6% in primary tumours and 100% in recurrences) and 82.1% (83.3% in primary tumours and 80% in recurrences), respectively. Radioimmunoguided surgery changed the surgical procedure in two cases with small tumour deposits. Occult regional lymph node involvement in primary tumours was not found; therefore, RIGS, as a complementary technique to RIS, is particularly useful in recurrences and can help the surgeon in the resection of small tumour deposits which are difficult to localize.