Immunocytochemical localization of serine:pyruvate aminotransferase (SPT) in rat hepatocytes was studied using a protein A-gold technique. Rat liver was fixed by perfusion. Vibratome sections (100 micron thick) of the liver were embedded in Epon or Lowicryl K4M. Ultrathin sections were incubated with antiSPT, followed by protein A-gold complex. Gold particles representing the antigenic sites for SPT were seen in three subcellular compartments, peroxisomes, mitochondria, and cytoplasm. In the control experiments the specificity of the immunolabelling was confirmed. Quantitative analysis of the labelling density showed that main subcellular compartments containing SPT are mitochondria and peroxisomes. In addition, the gold particles distributing in the cytoplasm were 16%-29% of the total labelling. The result indicated that the cytoplasm also contains SPT with a low density.