The methyl green assay method has been used to determine the activity of DNase I and DNase II in ram, bull, human, dog and rabbit seminal plasma. The activity of DNase II was higher except in the ram. The DNase I activity of ram seminal plasma was inhibited by low concentrations of citrate, zinc and cetyltrimethylammonium bromide ; this detergent also caused a reduction of DNase II activity. There was a stimulation of DNase I activity by 10 mm-citrate but not by concentrations less than 2 mm. Addition of antibiotics or shaking of ram seminal plasma with toluene did not affect the activity of either DNase. After deep freezing of ram semen there was a decrease of DNase I activity in the plasma and an increase of DNase I and II activity in extracts obtained from disintegrated spermatozoa. No significant change in DNase activity could be detected in the plasma of ram semen which had previously been subjected to cold shock.