Twelve male mice, 3 weeks old, were caged together with caries-active hamsters and the twin brothers of the mice served as controls. The hamsters harboured Streptococcus mutans and Streptococcus salivarius in their mouths, while neither of these streptococcal species could be recovered from the mice at the beginning of the experiment. At the end of the experiment after nine weeks Strep. mutans could be isolated in a high number from the mouth of six mice reared in the company of hamsters. Five of these mice had a moderate caries activity (2–19 lesions) while one was caries-free. Of the six mice from which Strep. mutans was not recovered, one mouse had five caries lesions while the other five animals were caries-free. In the 12 control mice Strep. mutans could not be isolated and only one animal had one caries lesion. This experiment demonstrates the transfer of Strep. mutans from hamster to mouse and a possible relation between the presence of Strep, mutans and caries activity in the mous