The potential proteolytic ability of cariogenic streptococci was assessed in a strain of Streptococcus mutans (GS-5). Blood-borne S. mutans from rats infected by mouth with GS-5 were cultured, and sterile rat tail tendons were placed on these cultured bacterial colonies. The collagenolytic ability of these organisms was examined under the scanning electron microscope and by testing for reduced tensile strength of the collagen fibers. These experiments showed that GS-5 cariogenic streptococcus can be conditioned to produce significant collagenolytic activity.