Cell mediated immunity to ultrasonicates of the dental plaque bacteriaBacteroides melaninogenicus, Fusobacterium nucleatum, Veillonella parvula, andActinomyces viscosusand to PPD was studied in edentulous controls and patients with periodontal disease by means of the leukocyte migration test in agarose. In patients with gingivitis, migration inhibition was significant toV. parvulaonly, but correlated with the degree of plaque accumulation and gingival inflammation. In patients with moderate periodontitis the migration inhibition response was further increased and was significant with all four plaque antigens. However, in patients with severe periodontitis or juvenile periodontitis migration inhibition was not significant and was apparently suppressed. The response to PPD was of the same magnitude in controls and all four groups of patients indicating that the observed migration inhibition reactions to the dental plaque bacteria are controlled by specific factors. It is suggested that the depressed leukocyte migration inhibition response in patients with advanced periodontal destruction is due to an exhaustion of immune cells in peripheral blood.