Serum alpha-fetoprotein (AFP) has been measured during chemotherapy of ten patients with hepatocellular carcinoma. Whenever the AFP concentration of a sample was lower than that of the previous sample the apparent half-life (AHL) of the protein was calculated. The biological half-life of AFP is 5 days so that values in excess of this were indicative of continuing or increased synthesis and secretion of AFP. The AHL for AFP provided a means of assessing efficacy of treatment. Increases in AHL generally predicted a rise in serum AFP and give advance warning of the need to change therapy.