The toxicity of mercury on hepatocytes was studied at the ultrastructural, biochemical, and immunocytochemical levels. Albumin metabolism was examined because it is a representative liver‐specific function. A novel cytochemical method using the protein A‐gold technique for the in situ localization of albumin in hepatocyte cultures was applied. Primary rat hepatocyte cultures were exposed to increasing HgCI2concentrations. Cytotoxicity was assessed by measuring the release of lactic dehydro‐genase from the cells. At the highest exposure concentration tested (50 μM), Hg was found to be significantly cytotoxic in contrast to what occurred at 5.0 and 0.5 μM. The level of albumin secreted, as measured by ELISA, was decreased by approximately 38% at 5.0 μMHgCI2and was found not to be different from that of controls at lower concentrations. The ultrastructural analysis showed that hepatocytes treated with 5.0 μMHgCI2undergo drastic morphological changes such as a decreased number of ribosomes associated with the rough endoplasmic reticulum, and the disappearance of the latter organelle, proliferation of the smooth endoplasmic reticulum, and dilatation of both the Colgi apparatus and the biliary canaliculus‐like structures. Immunocytochemical detection of albumin‐immunoreactive sites using protein A‐gold labeling further revealed that these were less abundant in hepatocytes treated with 5.0 μMHgCI2(—64%) as compared to control preparations. These results suggest that one of the effects of mercury on hepatocytes is to affect liver‐specific functions such as albumin production, possibly through interference with ribosomal function. This study also demonstrates for the first time the applicability of the high‐resolution protein A‐gold technique for toxicological investigations on hepatocytes in vitro.