Sixty children treated for solid tumors with high-dose chemotherapy followed by bone marrow transplantation were randomly assigned to one of two antibiotic protocols. Group A received prophylaxis consisting of ceftazidime plus teicoplanin beginning before the onset of aplasia and fever; group B received exactly the same antibiotic regimen but beginning at the onset of fever. The two groups were compared in terms of the rate of septicemia, fever of unknown origin, the time-lapse before the appearance of septicemia, the sensitivity of the causative organisms to the antibiotics, the effect of the latter on the intestinal flora, and the rate of fungal infections. The incidence of septicemia was significantly lower in group A (6.6%) than in group B (24.0%), mainly due to the prevention of episodes of early onset. Similarly, the appearance of the first episode of fever was delayed in group A, and the overall duration was reduced. Amphotericin B was prescribed empirically with the same rule in both groups, but three children in group A did not require amphotericin B. The effect on the intestinal flora was similar in the two groups; it must, however, be closely monitored so that the presence of potential pathogens can be dealt with appropriately.