Capsular structures of Porphyromonas gingivalis have been correlated to the pathogenicity in animal models. Six polysaccharide capsular serotypes have recently been described in P. gingivalis. In the present study, virulence of the P. gingivalis strains of the six capsular serotypes was compared with strains of the non-capsular serotype of P. gingivalis in a mouse model. All 18 encapsulated strains caused a spreading type of infection with exudate, often accompanied by ulceration and necrosis of the skin and a significant loss of body weight. All three non-encapsulated strains tested induced localized abscesses, although a spreading type of infection was occasionally observed. Illness of the mice was almost always associated with recovery of encapsulated P. gingivalis from blood, kidneys, lungs and spleens. A statistically significant contrast was found in the average weight change during the experiment between the mice infected with the K- serotype strains and the mice infected with the encapsulated P. gingivalis strains. Differences in severity of illness and the recovery of P. gingivalis from mouse tissues were recorded between strains of the same serotype. The differences in virulence within a capsular serotype suggest that the capsule is an important but not the only determining virulence factor for P. gingivalis.