Anti-oxidants are essential for intracellular free radical scavenging. as free radicals are one of the causes for tumorigenesis. Our objective Was to use bilirubin and investigate its action on human carcinoma cell lines. Bilirubin manifested as a prooxidant showing its cytopathic effect on TMK-1, showing growth inhibition close to 50%. Cell cycle analysis showed an arrest at G0/G1. Flow cytometry investigations with Red CC-1 showed an increase by more than 2 times Suggesting a prooxidative role of bilirubin. To check the effect of radicals on DNA, a Comet Assay displayed a typical comet's tail with bilirubin treated slides, only. Further, staining with DAP1 showed apoptotic action of bilirubin. Decreased mitochondrial function by bilirubin was observed with Mitotracker Green FM staining. These unexpected data have led us to conclude that bilirubin has anti-cancer activity as a prooxidant and may have a more vital role in the human body than realized. (c) 2006 Elsevier Inc. All rights reserved.