The correlation between the cytochrome a(+a 3)turnover number in phosphorylative capacity and cytochrome a(+a 3) content was studied in liver mitochondria from various patients. Mitochondrial function was classified into three types according to the turnover number: (1) normal type (a turnover number of 20 to 35 times per second); (2) enhanced type (a turnover number of 35 to 45 times per second); (3) depressed type (a turnover number of 0 to 20 times per second). In the enhanced type of mitochondrial function, the respiratory control ratio and the P:O ratio were considerably higher than those in the normal type, which suggests that the mitochondria were in the condition enhanced by an increased metabolic load on liver. Conversely, in the depressed type of function the cytochrome a(+a 3) content was decreased to less than 0.5 × 10 −10 mol mg of protein, with considerably lower respiratory control and P:O ratios. The mitochondria could not respond to energy needs with a normal acceleration of oxidative phosphorylation. Patients with such depressed mitochondrial function showed a high surgical mortality and complication rate. Our results indicate that considerations of the critical level of cytochrome a(+a 3) could provide a biochemical basis for judging the potency of compensatory and regenerative capacities of the liver.