The occurrence of Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis and Prevotella intermedia in subgingival plaque in 24 juvenile periodontitis patients was determined using DNA probe. 36 samples of subgingival plaque from 36 pockets having > or = 6 mm depth, > or = 3 mm of loss of attachment, and bleeding on probing and/or suppuration were taken from 18 patients with localized juvenile periodontitis (LJP, age range 12-24 years); and 12 samples from 6 patients with generalized juvenile periodontitis (GJP, age range 23-26 years). As control, an equal numbers of samples from healthy sites in the same patients were studied. P. gingivalis was found in 17 of 18 LJP patients, and in 31 of 36 diseased sites in those patients. P. intermedia was found in 15 out of the 18 LJP patients and in 28 of the 36 diseased sites. A. actinomycetemcomitans was present in 7 of the 18 LJP patients, and in 9 of the 36 diseased sites, and was not found in any GJP patients. All GJP patients had P. gingivalis (11 out of 12 diseased sites) and P. intermedia (all of the diseased sites). None of the three bacterial species was detected in healthy sites of GJP patients, and were found in healthy sites in only 2 of 18 LJP patients. The high prevalence and high levels of P. gingivalis and P. intermedia found in the LJP and GJP patients studied, suggest that there are populations affected by juvenile periodontitis in which this type of periodontitis is more associated with these species than with A. actinomycetemcomitans.