In gnotobiotic tests in rats, two xylite-degrading streptococcal strains were tested with regard to their cariogenic properties. One strain was isolated from human dental plaque, the other from the oral cavity of rats. The test animals were associated with the test strains only, and received a saccharose, fructose or xylite chocolate diet for 12 weeks. At the end of the test, between 10(4) and 10(5) inoculated germs were found in the plaque of the test animals. The total caries incidence in the gnotobiotic rats was insignificant in all test groups, i.e. whether they were given saccharose, fructose or xylite chocolate diet, and in both test strains. As compared to the cariogenicity of an S. mutans reference strain, the cariogenic effect of the tested strains was without any practical importance.