Using an indirect immunoperoxidase technique on frozen sections with the monoclonal antibody 96.5, we investigated the in situ distribution of melanotransferrin, a transferrin (Tf) and transferrin receptor (TfR) related glycoprotein, in human liver. Specimens included normal liver, liver in iron overload, hepatocellular carcinoma, angioma and foetal liver. On light microscopy, immunoreactivity was almost exclusively present on sinusoidal lining cells, apparently endothelial cells; the pattern was similar in normal and in iron-loaded liver. A gradient of more enhanced staining in acinar zone II and III was observed. The endothelial localization of the staining was supported by the positivity of the central vein endothelium and of the angiomas. Immunoelectron microscopy on three liver specimens showed positivity on sinusoidal endothelial cells but not on Ito and Kupffer cells. In addition, positivity on rough endoplasmic reticulum vesicles of some hepatocytes was also present. Four hepatocellular carcinomas showed an intense staining in tumour cells, 3 were weakly positive and 3 were negative. In the foetal livers, the central vein endothelium was positive from 21 weeks of gestation onward and additional positivity of zone III sinusoidal endothelial cells was present from 27 weeks on. The present results show that in the liver melanotransferrin has a localization different from Tf and the TfR. These latter molecules are predominantly localized in parenchymal cells. In addition, there does not appear to be a coordinate regulation secondary to iron storage, between melanotransferrin, Tf and the TfR. The observed gradient in the staining pattern in foetal and adult liver specimens further supports the heterogeneity of the endothelial cell population in the liver and suggests a developmental relationship between endothelial cells of sinusoids and central vein.