A number of Streptococcus mutans AHT mutants which vary in their in vitro adherence ability have been mutagenically isolated in our previous study. When grown in sucrose broth, mutants, M1, M2 and M13 fail to form adhesive deposits on glass surface. Mutant M9 produces a very small quantity of strongly adhering deposits, whereas M35 forms a large amount of loosely adhering ones. In this study, these mutants were examined for their plaque-forming ability and cariogenicity in an experimental caries model. Weanling albino hamsters pretreated with penicillin in drinking water were reared on a cariogenic diet #2000 during the experimental periods. Oral inoculations of 1-day cultures of the test strains were performed in the early stage of or at weekly intervals throughout the period. Plaque and caries on molars were evaluated. Parent WT strain produced heavy plaque and carious lesions on molars. On the contrary, the nonadhering mutants, M1, M2 and M13, and the in vitro adherence ability-decreased mutant M 9 could never be recovered from the mouth of animals even after the repeated inoculations. Mutant M 35 increased in the adherence ability established on molars, but recovery of the mutant was extremely low throughout the experimental period. None of the mutants, including M 35, significantly produced plaques and caries in hamsters.