DNA polymerase activity from Escherichia coli can be demonstrated in various sized molecules ranging in molecular weight from about 10,000 to 120,000 or higher. The characterization of the smaller species is difficult because of their pronounced tendency toward aggregation; the smallest apparently aggregates most readily. The results indicate the following molecular weight classes: 10,000-20,000; 40,000-50,000; 75,000-85,000, and 100,000-120,000. The same classes were obtained with several methods of analysis of material that had been purified in a number of ways, one of which is a new DNA-acrylamide gel chromatographic procedure. The lowest molecular weight species shows no exonucleolytic activity. A proteolytic inhibitor, phenylmethyl sulfonylfluoride, did not eliminate the small active molecules, although proteolysis of high molecular weight DNA polymerase (109,000) has been shown by others to produce fragments of about 75,000 molecular weight. Either there is a naturally occurring polymerase protein of about 20,000 molecular weight, capable of aggregation with itself and with certain other molecules (e.g., exonucleases), or there are certain bonds in a large, native polymerase molecule that are especially susceptible to proteolysis without destroying activity.