Tissue-engineering approaches for cartilage repair hold promise for the treatment of cartilage defects. Various methods to prevent or reduce dedifferentiation during chondrocyte expansion are currently under investigation. In the present study we evaluated the effect of oxygen on chondrocyte proliferation, as oxygen has received increased attention as a possible regulator of chondrocyte differentiation and its effect during expansion is uncertain. Therefore, the effect of three oxygen tensions (4, 10.5, and 21%) was investigated in a bioreactor microcarrier culture, which allows precise control of the oxygen tension in the liquid phase. During culture cells acquired a round shape on microcarriers. No differences in proliferation rate of chondrocytes were observed within the range of oxygen tensions evaluated. Cells exhibited predominantly anaerobic metabolism and, per mole of glucose, approximately 2 mol of lactate was produced independent of oxygen tension. Cellular oxygen consumption was comparable for all bioreactor cultures. Nevertheless, specific consumption rates were relatively high (2-4 x 10(-17) mol. cell(-1). s(-1)), in comparison with chondrocytes in cartilage (0.8-2.2 x 10(-18) mol. cell(-1)). Subsequent cartilaginous tissue formation in pellets was not affected as qualitatively assessed by safranin-O staining. At the oxygen concentrations evaluated, no effect of oxygen tension was observed on proliferation, oxygen consumption, and yield of lactate on glucose administration. For future investigations of chondrocytes and oxygen, the bioreactor system, which allows precise control and monitoring of oxygen tension, holds promise.