Neutrophil function was assessed in family in which only one member suffered from Juvenile Periodontitis (JP). Directed mobility (fMet-Leu-Phe and zymosan-activated plasma) was decreased in all siblings without involving a seric inhibitor. Adhesion was studied by a new method which allows for the evaluation of both adhesive rate and binding strengths. The latter parameters were decreased in the parents neutrophils, but remained increased in a set of twin sisters. The specific receptor induced phagocytosis was altered in all members of the family (FC receptor: IgG-SCR, C3b and mannosylfucosyl receptor: zymosan). The superoxide generation in response to fMet-Leu-Phe was decreased while the PMA response was almost normal. These results suggest an overall abnormality of ligand-receptor interactions (C5a, fMet-Leu-Phe, Fc and C4b receptors), this defect seems to involve some membrane characteristics and underlines the absence of correlation between PMN deficiency and the clinical expression of J.P.