Germ-free rats, fed a modified cariogenic diet 2000 with a reduced sucrose content of 25%, were inoculated with a strain of Streptococcus mutans, Streptococcus intermedius or Propionibacterium acnes, respectively. After 100 days on the diet, the rats were sacrificed. Microscopic examination of stained decalcified tooth sections revealed marked destruction of the outer zones of the dentin infected with S. mutans. The destruction was closely associated with the formation of glucan-mediated large aggregates of S. mutans. The aggregates, however, were rarely formed in the deep zones of the infected dentin. Inoculation with S. intermedius and P. acnes had little effect on the microscopic outline of the decalcified tooth sections. However, the staining behavior of the sections with Mallory's method, Alcian blue-PAS and silver was like that of the tooth sections infected with S. mutans, even in the central and inner zones of the dentin. The histochemical alterations of the deep zones of the dentin induced by the three infected bacteria appeared to be due to their proteolytic rather than to their acidogenic activities.