Cockayne syndrome (CS) is a human disease characterized by sensitivity to sunlight, severe neurological abnormalities, and accelerated aging. CS has two complementation groups, CS-A and CS-B. The CSB gene encodes the CSB protein with 1493 amino acids. We previously reported that the CSB protein is involved in cellular repair of 8-hydroxyguanine, an abundant lesion in oxidatively damaged DNA and that the putative helicase motif V/VI of the CSB may play a role in this process. The present study investigated the role of the CSB protein in cellular repair of 8-hydroxyadenine (8-OH-Ade), another abundant lesion in oxidatively damaged DNA. Extracts of CS-B-null cells and mutant cells with site-directed mutation in the motif VI of the putative helicase domain incised 8-hydroxyadenine in vitro less efficiently than wild type cells. Furthermore, CS-B-null and motif VI mutant cells accumulated more 8-hydroxyadenine in their genomic DNA than wild type cells after exposure to gamma-radiation at doses of 2 or 5 Gy. These results suggest that the CSB protein contributes to cellular repair of 8-OH-Ade and that the motif VI of the putative helicase domain of CSB is required for this activity.