Quantitative and qualitative examinations of the predominant cultivable microflora of the advanced tissue destruction (pocket) associated with juvenile periodontitis and of control pockets without loss of attachment were carried out by using anaerobic tube culture technique. The microflora from the control sites consisted primarily of streptococci, Gram-negative rods, and Gram-positive rods. In contrast, the test pockets had a relatively stable and characteristic microflora, dominated by various Gram-negative, anaerobic rods. The Gram-negative rods were divided into main groups, but a satisfactory classification of many of these microorganisms could not be performed with the methods used and the data available. The predominating Gram-negative rods differed from individual to individual. The role of the Gram-negative rods in the etiology of juvenile periodontitis is unknown. Some potential pathogenic mechanisms of these microorganisms are discussed.