To determine the significance of the presence of intragranulocytic micro-organisms in the blood buffy coat in patients with suspected infection, buffy coat examination and blood cultures were simultaneously performed in 455 consecutive patients with fever. There was no general correlation between the finding of intragranulocytic micro-organisms in the buffy coat and positive blood cultures. Patients with persistent bacteraemia and sterile blood cultures were, however, shown to have persistently positive buffy coat findings on repeated examination. These patients, who had culture-negative endocarditis or catheter-associated infections, had sterile blood cultures because of antibiotic treatment. Repeated positive findings in the buffy coat may therefore be valuable in detecting patients with persistent bacteraemia, but sporadic findings of micro-organisms in the buffy coats of acutely ill patients seem to have little diagnostic value.