The addition of Mg++ in a final, optimal concn. of 0.003 [image] to human serum leads to distinct and readily measurable desoxyribonuclease activity. The concn. of Mg ion necessary for optimal activity of guinea pig, rabbit and dog sera was similarly found to be 0.003 [image]. The enzymatic activity of human serum is thermolabile and independent of pH between 7.5 and 7.8. The avg. value of this activity in a group of 50 patients with cancer was significantly lower than the avg. in a group of 34 patients, of comparable age, with noncancerous disease. Both avg. values were less than that of a normal group. No explanation for the occurrence of decreased serum desoxyribonuclease activity in disease can be offered at present.