Having found raised serum calcitonin concentrations in 94% of patients with hepatocellular carcinoma when using a dextran-coated-charcoal radio-immunoassay, we have now repeated the study, using a double-antibody radio-immunoassay, in 102 further patients with hepatocellular carcinoma and 35 matched controls. Serum immunoreactive calcitonin concentrations (iCT) in the controls ranged from 10 to 310 pg/ml (mean 154.6 pg/ml). Values in the tumour patients ranged from 10 to 1,650 pg/ml (mean 302.6 pg/ml). The mean figures were significantly higher in the tumour patients (P less than 0.001), 35.5% of them having values above 310 pg/ml. In 65 of the patients serum iCT concentrations were also determined by dextran-coated-charcoal radio-immunoassay. Values ranged from 10 to 10,780 pg/ml (mean 2,179 pg/ml). If 1,000 pg/ml is taken as the upper limit of normal, 69% of the patients had raised iCT concentrations. There was a good correlation (r = 0.67; P less than 0.001) between serum iCT values measured with both methods in 50 patients. If measured by the double-antibody radio-immunoassay method, the serum calcitonin value is not useful as a marker for hepatocellular carcinoma.