Rat kidney, spleen, brain, and liver DNA-methylases were partially purified by chromatography on DEAE-Trisacryl columns and their catalytic properties were studied. Crude extracts contain one or several inhibitors which are thermostable and resistant to acidic or alkaline treatments and which can be eliminated by dialysis, or by chromatography on DEAE-Trisacryl. These are most probably divalent ions, such as, Pb2+, Zn2+, Cu2+, Fe2+, Mg2+, Mn2+ or Ca2+, which inhibit the DNA-methylase activity. However, Co2+, at concentrations ranging from 0.05 mM to 1 mM, has an efficient stimulatory action on spleen, kidney or brain DNA-methylase activity. The spleen DNA-methylase activity on chicken erythrocyte DNA could be increased 10-fold, by a 0.2 mM concentration of Co2+, but no stimulation was found with liver DNA-methylase. The fact that significant differences exist between the DNA-methylases from the different organs in their behavior towards Co2+ could indicate that these enzymes are different.