Summary.Gnotobiotic rats mono‐infected withStreptococcus mutans(NCTC 10919) were maintained on a high sucrose diet in an isolator. When they were 8 weeks old the pulps of their maxillary first and second molars were mechanically exposed and left open to the oral environment. The rats were sacrificed in groups after 7 and 28 days. Thirty‐seven teeth were available for study in the 7‐day group, and 30 teeth in the 28‐day group. The testS. mutanswas non‐cariogenic. After 7 days, necrosis of the superficial layers of the pulp was noted; however, after 28 days pulpal necrosis was much more extensive and calcific repair was observed in only one‐third of the teeth examined. The microorganism did not provoke inflammation in the pulp or periapical tissues. This is the second strain ofS. mutanstested in this manner. These results are very similar to those reported previously forS. mutans(NCTC 10449) and strengthens the view that organisms of theS. mutansgroup have a low pathogenicity f