Phagocytosis is a major mechanism of defense against bacterial infections. The ingestion of different microorganisms by blood granulocytes or monocytes may involve a variety of cell membrane recognition structures (e.g., immunoglobulin or complement receptors, lectin-like structures or other nonspecific binding sites). It is of interest to know which mechanism plays a prominent role in the management of a particular type of infection.Forty-three pathogenic bacterial suspensions were obtained from patients under mechanical ventilation at the onset of nosocomial lower respiratory tract infections. They were coincubated with blood granulocytes from the same or other patients in the presence or absence of the corresponding serum. Phagocytosis and the oxidative burst were then assayed. We conclude the following: a) Substantial phagocytosis was found under serum-free conditions and the patients' sera did not dramatically enhance bacterial uptake during the first days after the onset of clinical symptoms, b) The phagocytes from an infected patient did not display any peculiar inability to bind to the bacteria that grew successfully in this subject. Hence, the occurrence of a particular infection might be dependent on a defect of intracellular killing of ingested pathogens or on the conditions of infection development.