Elevated Serum B12 levels were found at diagnosis in five of eleven patients with primary hepatocellular carcinoma. During chemotherapy either with CB3717 or VP16 the serum B12 rose dramatically, in one case reaching levels ten times the upper limit of normal. However, the serum LD activities did not change in parallel with the serum B12 levels suggesting that there was little necrosis of the tumour or the liver. With two out of five patients with other types of cancer the serum B12 levels also increased but less markedly. This data seems to suggest that the serum B12 level may not be as good a tumour marker for hepatocellular carcinoma as has been suggested and indeed may be influenced by the chemotherapeutic agent.