The enzyme calf thymus polymerase requires denatured or single-stranded DNA as a primer for DNA synthesis and is inactive on native DNA preparations. The enzyme and tritium-labeled deoxyribonucleoside triphosphates were incubated with alcohol-fixed and Carnoy-fixed tissue preparations to see if primer DNA could be found in several types of cells undergoing DNA synthesis. In all cases, low-pH controls were prepared for comparison. Priming activity was not found in nuclei that had been fixed in alcohol. Priming activity was found in cell nuclei that had been fixed with an acid fixative or had been treated at a low pH prior to treatment with the enzyme reaction mixture.