Abstract: A method of assessing the in vivo state of phagocytes in a patient, possibly indicating diagnostically important states such as inflammation or infection, which method utilizes chemiluminescent (CL) light emitted during the reaction in vitro between a CL substrate and the reactive oxygen species (ROS) formed in a fluid sample obtained from the patient. The measurement is performed in two or more portions of the sample, with stimulated phagocytes affected by one or more priming agents which shift the functional state of the phagocytes, providing a plurality of measurements, which are analyzed so as to distinguish intracellular and extracellular contributions to the CL kinetics. The results are compared with a range of control measurements performed with patients suffering from various diagnostic conditions.