High-GC isolates of P. putrefaciens undergo extensive autolysis after growth, resulting in a marked decrease in turbidity and the release of high-molecular-weight DNA which imparts a high viscosity to culture broths. The native DNA released is resistant to attack by the exocellular DNase activity of the culture broths. Autolysis is inhibited by a pH of 6.0 and the presence of 0.001 M Mg++ or Ca++, and is enhanced by elevated pH values and temperatures. This autolytic phenomenon in broth cultures readily distinguishes high- from low-GC isolates. The latter do not exhibit autolysis.