Previous studies from our laboratory cytochrome au3 in human liver increases have shown that the concentration of in accordance with the severity of liver cirrhosis [l], and that the oxidase concentration serves as a useful index to decide whether or not massive hepatic resection is possible [2]. The spectrophotometric determination of cytochrome au3 concentration in human liver, however. requires > 1 g liver tissue since mitochondria have to be isolated, hence it can only be performed intraoperatively. In addition, this assay takes > 2 h, especially with cirrhotic liver specimens, due to the time required to homogenize the tissues. Thus we have sought an alternative method of rapid and simple assay of cytochrome au3 concentration. Recently, we found that the chronic subcutaneous administration of carbon tetrachloride to rats resulted in an increase in the concentration and the specific activity, or V,,,, of cytochrome c oxidase (ferrocytochrome c : oxygen oxidoreductase, EC 1.9.3.1) as well as a slight increase in K, for cytochrome c, as compared with normal rats [3,4]. In this study we describe a method of measuring the cytochrome c oxidase activity of tissue homogenates from human liver specimens obtained by needle biopsy, and show that this assay procedure could replace the hitherto adopted spectrophotometric determination of cytochrome au3 in isolated mitochondria for clinical purposes.