It has been previously demonstrated that the conditioning therapy given to bone marrow transplant (BMT) recipients creates a high oxidant stress, resulting in a measured reduction in antioxidants, such as glutathione peroxidase (GSH-Px), vitamin E, and cell peroxide fragilities. As part of a current intervention trial of antioxidant therapy in BMT recipients, plasma thiobarbituric acid reactive substances (TBARS) were measured to assess peroxidation and free radical activity. Measurements were performed before and after conditioning therapy, and then at weekly intervals for a period of 6 wk after transplantation in 20 patients (10 controls and 10 antioxidant therapy [AOT] recipients). The TBARS results were compared with concurrent measurements of more specific elements of the antioxidant pathways, such as red blood cell glutathione peroxidase (RBC-GSH-Px), plasma vitamin C, and serum vitamin E. In all cases, TBARS concentration was significantly increased after conditioning compared with baseline levels (p < 0.001), an increase that correlated inversely with RBC-GSH-Px (r = -0.81; p < 0.01). The TBARS concentration fell gradually after conditioning in all patients. The fall in the AOT group was more rapid than in the control group, and it paralleled the gradual return toward normal levels of the other antioxidants. The change in TBARS concentration occurred faster than changes in other indices, suggesting that TBARS might be a better index of overall free radical activity. Although the patient numbers are small, there is some evidence to suggest that MDA may act as a prognostic marker.(ABSTRACT TRUNCATED AT 250 WORDS)