The study was undertaken to define the potential use of indium 111 carcinoembryonic antigen-specific antibody labelled [CEA F(ab')2] for the radioimmuno-detection of colorectal carcinoma using an intraoperative hand-held gamma probe. The use of a linear radioactive source allowed optimization of physical characteristics. The best results regarding sensitivity and resolution were obtained using a 5-mm thick tungsten alloy collimator. A simulation study with a liver phantom (22 MBq or 0.6 mCi) was performed to determine the effect of side scatter as opposed to direct background and showed that it is possible to detect small radioactive targets (3.7 KBq or 0.1 mu Ci) 4 cm from the phantom. A clinical study performed with ten patients showed that tumours with good uptake of CEA-specific antibody could be detected with sufficient contrast in two patients when the probe was used. Results of a biodistribution study performed after tumour fragment or normal tissue countings in a well counter showed high tumour uptake (above 8 x 10(-3) injected dose/g) and tumour-to-normal tissue ratios (between 2.5 and 20) in five patients. Results with the probe showed markedly lower ratios. There was no correlation between absolute tumour uptake and the count rates of tumour measured intraoperatively. This can be attributed to the degradation of depth resolution resulting from the high energy photopeak of gamma-emitting 111In.