A system for the extracorporeal treatment of end-stage liver failure patients has been developed and is now in clinical studies. This HepatAssist 2000 system consists of perfusing the plasma of the patient through a hollow-fiber bioreactor containing primary porcine hepatocytes. The goal of the system is to assist the patient while the liver regenerates or a suitable liver is found for transplantation. During the development of this system in the laboratory, an experimental protocol has been developed to simulate patient treatments. The protocol substitutes donor calf serum for patient plasma, but is otherwise similar to clinical conditions. The protocol uses a blood gas analyzer to monitor dissolved oxygen and carbon dioxide tensions at the entrance to and exit from the bioreactor. Samples are withdrawn using a standard 1-ml syringe and analyzed off-line immediately. The blood gas measurements have been used to verify that the oxygenator, placed immediately prior to the bioreactor in the plasma circuit, is properly sized to allow complete equilibration with feed gas and to maintain a physiologic level of carbon dioxide. The measurements were also used to calculate overall bioreactor oxygen consumption rates under various conditions. In one set of studies, composition of the feed gas to the oxygenator was varied from 20% to 70% oxygen, while maintaining 5% carbon dioxide and balance nitrogen. Rates of oxygen consumption and liver cell function (metabolism of diazepam) were unchanged, and no signs of oxygen toxicity were observed. In another set of studies, oxygen consumption increased linearly with number of hepatocytes used in the clinically used range of 5-10 billion hepatocytes, whether all hepatocytes were placed in one device or split between two devices operating in series or in parallel. In a third set of studies, oxygen consumption was a weak function of flowrate through the bioreactor but was highest at the clinically used recirculation rate of 400 ml/min. These data indicate that blood gas meters--readily available in hospital clinical laboratories--can be used to assist in the design of extracorporeal cell therapies, to monitor the effectiveness of system components, and to assess the effects of system modifications. These same measurements can be made noninvasively in the clinical setting to monitor oxygen consumption rates during patient treatment.