The thiobarbituric acid (TBA) assay was applied to uremic blood. Significant TBA-reactive material was found in patients with chronic renal failure compared to normal controls. The presence of TBA-reactive material correlated with the degree of renal failure, and was removed by hemodialysis. The reactive material is not 2-deoxyribose or sialic acid. In addition, the presence of TBA-reactive material in uremic blood is not abolished by prior aspirin ingestion, suggesting it is not related to active prostaglandin synthesis. The possible origin of the TBA-reactive material in uremic blood is discussed.