The present invention provides a method for reducing the risk of bacterial infection or sepsis in a susceptible patient by treating the susceptible patient with a pharmaceutical composition containing bacteriophage of one or more strains which produce lytic infections in pathogenic bacteria. Preferably, treatment of the patient reduces the level of colonization with pathogenic bacteria susceptible to the bacteriophage by at least one log. In a typical embodiment, the susceptible patient is an immunocompromised patient selected from the group consisting of leukemia patients, lymphoma patients, carcinoma patients, sarcoma patients, allogeneic transplant patients, congenital or acquired immunodeficiency patients, cystic fibrosis patients, and AIDS patients. In a preferred mode, the patients treated by this method are colonized with the pathogenic bacteria subject to infection by said bacteriophage.