The effect of cyclosporin-A (CyA) on the dentogingival flora of ferrets with healthy and experimentally induced periodontal breakdown was studied. Five animals were given 10 mg/kg/d CyA. At the start of the experiments (day 0), ligatures were placed around 4 teeth in the right upper and lower jaws; corresponding contralateral teeth on the left side served as control. On days 0 and 28 (end of the experiment), microbiological samples were collected from the gingival sulcus of the experimental and the control teeth and from closely located gingival mucosa membrane. The samples were subjected to viable counts and to darkfield microscopic analyses. On day 0, facultative anaerobic rods, mainly Pasteurella spp, Alcaligenes spp, Corynebacterium spp. and Rothia spp dominated in the viable counts. No anaerobic bacteria were detected in the viable counts. On day 28 spirochetes increased in the experimental gingival sulcus samples and anaerobic bacteria appeared in most of the samples and constituted 40-60% of the total cultivable flora; Fusobacterium necrophorum and Eubacterium spp. predominated in the samples from the experimental sites. The results of the present study were compared with those of our previous investigation of ferrets not medicated with cyclosporin but also subject to experimental ligature periodontitis. Eubacterium spp. were absent in the animals not treated with cyclosporin, while this species was frequently present in the immunosuppressed ferrets. The results indicate that the presence of the large numbers of gram negative rods and of anaerobic bacteria may have enhanced the inflammatory process and further provoked the gingival overgrowth observed.