In vivo determination of the labelling index of metastatic colon carcinoma was performed in 14 patients following intravenous infusion of bromodeoxyuridine (BrdU), at doses of 100 and 50 mg/M2. No toxicity was seen. Labelled cells were detected by an immunohistochemical technique using a monoclonal antibody to BrdU. Labelling was uniform and highly specific with good preservation of morphology. Labelling index values ranged from 15.0-40.6% for metastatic lesions and 3.6%-20% for normal colonic mucosa, with mean values of 24% and 11%, respectively. This represents a useful new method for in vivo labelling index determination in solid tumors.