Lipoprotein synthesis was demonstrated by double diffusion with low density lipoprotein antibody, and by 3H-labeled amino acid incorporation into proteins of the d less than 1.063 g/ml centrifugally isolated lipoprotein fraction. Radioactive label was incorporated predominantly into apolipoprotein B (60%), apolipoprotein A-I (20%) and apolipoprotein C (12%), as determined by Sepharose column chromatography and polyacrylamide gel electrophoresis. Incorporation of radioactive label into apolipoprotein B was inhibited by the presence of albumin in the medium, and was restored to control levels with the addition of 1 mM oleic acid, indicating that cell synthesis of apolipoproteins could be modified by culture conditions. The human hepatoma cell line, Hep G2, provides a potential in vitro model for the study of regulation of human hepatic lipoprotein and apolipoprotein synthesis.