To evaluate the susceptibilities of human blood constituents to the low levels of ozone used in ozonated autohemotherapy (40 microgO3/ml), we quantified plasma antioxidants and erythrocyte constituents after rapid mixing of human whole blood with ozone at 20, 40, 60, and 100 microg/ml blood. Ascorbic acid, uric acid, and alpha-tocopherol in plasma decreased as ozone increased, but bilirubin was unaffected. The content of thiobarbituric acid-reactive substances in plasma was increased by ozone. However, the content of thiobarbituric acid-reactive substances and alpha-tocopherol in the erythrocyte membrane was not significantly affected. No significant changes occurred in the content of methemoglobin, cytoskeleton proteins or erythrocyte enzymes such as Na+/K+-ATPase, acetylcholinesterase, catalase, glutathione peroxidase, glutathione reductase, and superoxide dismutase at all the ozone levels tested. A decrease in reduced glutathione in erythrocytes was the only significant change caused by the ozone level used for autohemotherapy. It may be one of the chemical events responsible for the beneficial effects of ozonated autohemotherapy.