The T‐cell triggered effector systems have been emphasized as the most important agents mediating periodontal disease in recent years. It was therefore tempting to investigate whether T‐cell suppression with cyclosporin A (CS‐A) had an effect on the establishment and progression of the disease in rats monoassociated with Actinomyces viscosus Ny 1. The experiment was carried out with two treatments (16 rats each) in two isolators. In one of the isolators CS‐A was added to the high sucrose diet 2000 aiming at a dosage of 15 mg per day and kg bodyweight. In the second isolator (control) the same diet without CS‐A was fed. At day 25, 38, 52 and 65 four animals from each treatment were removed from the isolators. Dental plaque and alveolar bone loss were evaluated. The cellular composition of blood and histological changes of periodontal lesions on a light and electron microscopic level were monitored. In addition antibody titers and the T‐cell suppressing effect of individual serum samples of all animals were determined.Despite clearcut treatment differences in antibody titers as well as an interference by the sera of CS‐A treated animals with the Con A dependent lymphocyte activation, bone loss was observed in both groups. Haematological and histological investigation revealed only minor differences between the two treatments. It is concluded that periodontal disease in monoinfected rats seems to be the result of a multitude of pathomechanisms and not to be strictly correlated with T‐cell dependent h