The transmission of Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis and mutans streptococci was studied between 4 married couples who suffered from advanced periodontitis. Of the 20 couples investigated, the 4 in which both spouses harbored A. actinomycetemcomitans and P. gingivalis were chosen for the transmission study. Three of these couples also harbored mutans streptococci. A. actinomycetemcomitans isolates (8-24 per subject) and mutans streptococcal isolates (5-23 per subject) were serotyped by immunodiffusion technique. For ribotyping, chromosomal DNA from A. actinomycetemcomitans isolates (4-5 per subject) and mutans streptococcal isolates (4-11 per subject) was digested with restriction endonucleases ClaI or BglI and HindIII or SmaI, respectively. P. gingivalis isolates (2-15 per subject) were ribotyped by using ClaI, BglI and SmaI. The blotted restriction fragments were hybridized to the plasmid pKK3535, which contains the rRNA operon of the E. coli chromosome. The spouses in 2 couples shared the same sero- and ribotypes of A. actinomycetemcomitans and S. mutans. P. gingivalis ribotypes were identical in 2 couples. The result suggests transmission of oral bacteria between spouses.