Peripheral blood polymorfonuclear leukocytes (PMN) are activated by the contact with opsonized bacteria. Metabolic activation of PMN is associated with a remarkable increase in the respiratory burst and generates high energy oxygen compounds which are responsible for the bactericidal activity of neutrophils and for their ability to produce luminol-dependent chemiluminescence (CL). The CL phenomenon is measured by an automated and computerized photoluminometer (Berthold LB950) in whole blood, stimulated with opsonized zymosan. This whole blood method of CL has been applied to the investigation of PMN functional defects in early-onset periodontopatic patients.