The clinical and immunological data from 4 patients with generalized prepubertal periodontitis are presented. The peripheral blood neutrophil chemotaxis was measured using zymosan activated sera as the chemoattractant. All of the 4 patients have shown depressed neutrophil chemotaxis compared to those of the healthy controls. Peripheral blood lymphocyte subpopulations were analysed by double-coloured flow cytometry using monoclonal antibodies for the receptors CD2, CD3, CD4, CD8, CD19, CD29, CD45RA+, 34,29dK, CD56. CD11b/CD18. Lymphocytes bearing CD3 receptors showed a significant decrease compared to those of the controls. Natural killer cells were lowered in 3 of the 4 cases. All of the patients showed a higher increase in CD11b/CD18 expression. The evaluation of CD11b/CD18 receptor in peripheral blood leukocytes may be of help explaining the rôle of neutrophils in the pathogenesis of the disease.