The effects of regimens on the prevention of infection in 42 adult leukemia patients receiving bone marrow transplantation was analyzed. Standard risk patients (transplantation in 1st remission of acute leukemia and chronic phase of chronic myelogeneous leukemia received marrow from HLA compatible sibling or autologous marrow) showed shorter febrile days than high risk patients (transplantation in more advanced stage of leukemia and transplantation from unrelated donor), 1.33 mean days vs. 4.93 mean days respectively. Poorer intake of non-absorved antibiotics resulted in higher rate of bacterial colonization in stool after transplantation. And that, the degree of gut sterilization correlated with the duration of febrile days during the period of less than 100/microliter peripheral neutrophil count in high risk patients. Thus, prophylactic regimens of infection in bone marrow transplantation should be considered according to the risk of patient, that is, more practical and complete prophylaxis in risk patients and more conventional one in standard risk patients.