Sepsis, despite all the advances of the last 100 years, is still the commonest complication in all branches of surgery. Polymorphonuclear leucocytes (polymorphs) hold pride of place in the host defences against bacterial invasion and are the major cellular constituent of pus. Their prime function is phagocytosis. This thesis describes an , attempt to develop a test of phagocytosis which is accurate but simple to perform. Acridine orange (AO) staining with fluorescent microscopy of polymorphs on coverslips was investigated and showed in-vitro that polymorphs could function in extremely unphysiological environments, while conversely, small quantities of some substances, such as ethanol, or phenylbutazone, could markedly inhibit this process. The AO method was compared with others using microscopy or the reduction of colony-forming units. All, including the AO method, were found to be laborious, time- consuming and suffered from subjective error. A chemiluminescent assay of phagocytosis was developed which, despite drawbacks, was objective and easy. With this, the in-vitro effects of drugs and antibiotics were assessed and studies undertaken on groups of surgical patients. These showed that phagocytosis by polymorphs from the venous blood of patients receiving aortobifemoral grafts for claudication, and other operations varied more in relation to the type or traumatic severity of surgery than to the presence of sepsis. Polymorphs taken from venous blood, subcutaneous mastectomy wounds, the peritoneal cavity or from pus, differed from each other in their abilities to engulf microorganisms. Noteworthy was the finding that polymorphs from pus were not "dead". From this research it seems that the role of the polymorph in the aetiology and pathogenesis of sepsis in individual patients is poorly understood. It is hoped that the chemiluminescent assay described in this thesis will prove of value in further work in this field.