The DNA polymerases of the following eukaryotic tissues were studied: regenerating rat liver, normal rat liver, rat thymus, normal mouse liver and Ehrlich ascites-tumour cells. In all cases two main polymerase forms are observed, one of mol.wt. 200000, preferring denatured DNA to native calf thymus DNA primer, designated type I, and the other, designated type II, of mol.wt. 100000, showing a variable and slight preference for native calf thymus DNA primer. Some catalytic properties of these polymerases are described. Nuclei have been isolated from some of these tissues by using two different buffer systems. The ionic composition of the isolation medium is found to affect greatly the amounts and types of polymerase that bind to the nuclei, and also affects the kinetic properties of the polymerases. The way the polymerases and nuclei change properties as the ionic composition of the buffers is changed suggests that ionic effects may be a significant factor in the control of DNA synthesis in vivo. These ionic effects also explain much of the previous confusion over the localization of specific DNA polymerases.

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