The distribution of deoxyribonuclease II (DNase II) in tissues and body fluids was examined in 12-week-old C3H/He mice. Activity was observed in most tissues and body fluids except erythrocytes and serum, but their levels were quite different among tissues. Activity was high in the spleen, salivary gland, and preputial gland, moderate in the liver, kidney, thymus, lung, heart, pancreas, seminal vesicle, coagulating gland and prostate and low in the brain, testis and muscles. Sex difference, males having a significantly higher DNase II activity level than females, was observed in salivary gland, kidney and urine.