Abstract: The molecular basis for the augmented production of alpha-fetoprotein is unknown. We have used in situ hybridization of alpha-fetoprotein cDNA to malignant hepatocytes to establish if increased serum alpha-fetoprotein concentrations are related to detectable steady-state levels of alpha-fetoprotein mRNA in hepatocytes. Tumor tissue from four patients with histologically confirmed hepatocellular carcinoma were examined, and the results compared to fetal liver. Northern blot hybridization for alpha-fetoprotein mRNA in tumor tissue was also analyzed. As expected a high number of grains was observed in fetal liver tissue, indicative of high levels of alpha-fetoprotein mRNA physiologically present during pre-natal development. Sections from all patients with high serum concentrations of alpha fetoprotein showed appreciable hybrid formation, which correlated semi-quantitatively with the serum concentrations. However, hybrids were not detected in a patient with a normal serum alpha-fetoprotein. The high alpha-fetoprotein mRNA levels in fetal and neoplastic liver suggest that gene transcription is the mechanism of alpha-fetoprotein production in malignancy, although the control of this mechanism remains speculative.