The in vitro functions of highly purified blood monocytes were studied in 11 patients suffering from acute bacterial infections (e.g. erysipelas, appendicitis, abscesses). Chemotaxis, superoxide-anion generation, and beta-glucuronidase release of the patients' monocytes in response to the receptor-dependent stimuli formyl-methionyl-leucyl-phenylalanine (FMLP), complement split product C5a, leukotriene B4 (LTB4), and opsonized zymosan particles were measured. All the patients were examined in a follow-up study during the course of illness. A group of 33 healthy volunteers served as control. The patients revealed a transient decrease in monocyte chemotactic migration in response to all stimuli between days 3 and 5 after onset of clinical symptoms. Superoxide-anion generation from patients' monocytes was found to be enhanced 3 days after impaired chemotaxis. Stimulated release of lysosomal beta-glucuronidase showed a decrease in the first days of the disease. However, spontaneous beta-glucuronidase release was enhanced between days 3 and 7 in the patients' monocytes. Serial measurements of monocyte responsiveness. These results indicate a distinct modulation of monocyte functions during the course of an acute bacterial infection. Changes in monocyte maturity and/or activation under inflammatory conditions may be responsible for these alterations in monocyte function.