The aim of the present work is to discover the origin of the post-operative infections presented by certain patients following surgery for pulmonary exeresis in a surgical unit that deals both with thoracic and general surgical patients (most commonly cancer surgery). A prospective epidaemiological investigation of 54 pulmonary cases was thus undertaken, lasting for a period of 6 months. This consisted in obtaining multiple microbiological specimens from the patient himself, but also from the patient's environment and from the hospital staff (more than 1000 specimens were examined). The study begins at the stage of entry of the patient to the hospital department and follows him through the operation theatre stage and the post-operative period in the intensive care unit. In the intensive care unit the patients will come into contact with other patients who have been admitted urgently. These cases may or may not be of a pulmonary nature. Among the 54 subjects studied, 19 presented with infectious post-operative complications that led to 2 deaths. In 9 cases the infection was due to auto-contamination. It would appear that the environment plays a very small rôle as a factor leading to the development of post-operative infection since very few of the specimens taken were postive and usually the staphylococci were of various different lysotypes and never corresponding to the lysotype of the infective organism. Some predisposing factors to infection were demonstrated: an association with alcoholism and chronic bronchitis, pre-operatory bronchial suppuration, and widespread pulmonary exeresis was found.