Incidences of duodenal tumor induced by oral administration of hydrogen peroxide (HPO) and catalase activities in the duodenal mucosa, blood and liver were correlated in C3H/HeN, C57BL/6N, (C57BL X C3H)F1 (B6C3F1) and hypocatalasemic C3H/Cbs mice. A solution of 0.4% HPO was given to the mice as drinking water for about 6 months. Incidences of duodenal tumor were 11.1% in C3H, 31.8% in B6C3F1, 100% in C57BL and 91.7% in C3H/Cbs mice. Catalase activities (10(-4) k/mg protein) in the duodenal mucosa, blood and liver were 5.3, 7.8 and 75.3 in C3H, 1.7, 7.7 and 62.8 in B6C3F1, 0.7, 5.1 and 40.7 in C57BL, and 0.4, 0.4 and 33.3 in C3H/Cbs mice, respectively. The coefficients of correlation (r) of catalase activities with the incidences of duodenal tumor in C3H, B6C3F1 and C57BL mice are -0.83 in duodenal mucosa, -0.98 in blood and -0.99 in liver. The results indicate that the incidence of duodenal tumor induction by HPO is dependent on the individual mouse strain and may be determined by the genes controlling the catalase activities.