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Patients in intensive care units have frequent and severe opportunistic bacterial pneumoniae, even if they were previously free from respiratory disease. A search was made in these patients for possible immune deficiencies. Granulocyte and monocyte phagocytic activities were studied separately in 17 patients with bacterial bronchopneumonia (male: 13, female: 4; age: 41 +/- 5 yr). The ability of three types of particles (opsonized zymosan, immunoglobulin coated and glutaraldehyde-treated sheep red cells) to trigger ingestion was measured. Cells were tested either in normal AB serum or in the presence of patient's serum. A substantial 40% decrease of the fraction of granulocytes ingesting zymosan was found in our experimental conditions (p less than 0.001). Activity with the other particles was not significantly altered. Patients' sera were at least as efficient as a pool of normal sera in opsonizing zymosan. Furthermore, no phagocytic inhibitor was found in the patients tested. Underlying mechanisms for these abnormalities remain unknown, but a better understanding of the aetiology of the altered bactericidal function of phagocytic cells is required before suggesting immunomodulating treatments.