The action of deoxyribonucleases I and II has been studied as a function of ionic strength and pH, in the light of the theory of the ionic control of biochemical reactions (P. Douzou and P. Maurel (1977) Proc. Nat. Acad. Sci. USA, 74, 1013-1015). The pattern of DNA degradation by the two enzymes fits the general principles of the theory. However, the activity of DNAase II, a dimeric, basic protein (pI = 10,2) appears to be scarcely modulated by variables such as ionic strength and pH. This is reminiscent of what was elsewhere observed with the system double stranded RNA-seminal RNAase (also a very basic, dimeric enzyme), and could, therefore, tentatively be correlated with the dimeric and/or the very basic nature of the enzyme protein.