As a result of an epidemiological survey on the periodontal status of young adults, a school was found with a relatively high number of subjects showing loss of attachment (18 out of 87). In 10 of these subjects the microflora of one pocket and of the dorsum of the tongue was investigated with special reference to black-pigmented species and Actinobacillus actinomycetemcomitans. Prevotella intermedia was the most prevalent species, being found in 9 patients. A. actinomycetemcomitans was isolated from 3 subjects and Porphyromonas gingivalis from 2 subjects. To determine whether cross-infection had occurred, the P. intermedia, P. gingivalis, and A. actinomycetemcomitans isolates were examined by restriction endonuclease analysis of the DNA after digesting with one or a combination of the endonucleases Pst I, Bam HI, and Hind III. Evaluation of the restriction endonuclease typing method showed it to be sensitive and useful for the typing of P. intermedia, P. gingivalis, and A. actinomycetemcomitans. Using this restriction endonuclease typing method, all isolates of A. actinomycetemcomitans, P. gingivalis, and P. intermedia obtained from single individuals showed different digest patterns. We conclude that in the present study bacterial cross-infection did not occur among subjects in this school.