Localization of Intrahepatic ICAM-1 was investigated by light (LM) and electron (EM) microscopy in rats with chemical-induced cancer by using Immuno-peroxidase method.[Materials and Methods] Male Fisher rats were studied. Diethylnitrosamine (DEN) was dissolved in distilled water at a concentration of 100 ppm and this solution was offered at libitum. The animals were sacrificedafter 8 (Group A) and 12 weeks (Group B). Liver tissues were fixed in acetone, and treated with microwaves for the observations. Detection of intrahepatic ICAM-1 by LM and EM was performed by Immuno-peroxidse method using monoclonal anti-rat ICAM-1 antibody. [Results] 1) In group A, the nuclei of hepatocytes were irregular in size and changes in the staining of the cytoplasm were observed. In group B, marked nodulation was observed and part of the nodules were considered atypically hyperplastic (AH). In addition, small cancer lesions (CA) were observed. 2) In group A, its levels in both the endothelial cells and hepatocytes were lower thanin the control group. In group B, the ICAM-1 in the CA and AH areas was observed in the cell membrane. Its levels in theseareas were much lowert han in hepatocytes and endothelial cells which were considered to be normal areas in same section. 2) In EM, the ICAM-1 was observed in the membrane of cancer cells and infiltrating cells. [summary] Pattern of expression of the intrahepatic ICAM-1 showed a clearly differenced between normal and CA and AH areas even in the same section and its level decreased as the intensity of cellular atypia increased.