In an attempt to prevent severe infections complicating serious hematologic diseases, decontamination procedures for such patients occupying a protected environment were evaluated in this study. Oral gentamicin, vancomycin and nystatin in combination (“GVN”) was used for intestinal sterilization. Each of 3 healthy volunteers took each drug of “GVN” according to our regimen in a bioclean room, being served with sterile food. Clinical application of “GVN” was made on 3 patientswith hematologic disorders; one with acute myelogenous leukemia who took “GVN” on 2 occasions during induction therapy, and one each with aplastic anemia and acute lymphocytic leukemia treated by bone marrow transplantation. This antibiotic combination was confirmed to be one of the most excellent regimens for intestinal sterilization on the basis of both laboratory and clinical investigations. On its clinical application, however, attention should be drawn to probable long-term survival of a small portion of intestinal flora despite “sterile” stool cultures, difficulty in suppressing fecal flora, notably fungi, on repeated courses of the regimen, selection or appearance of resistant strains, and, furthermore, increased absorption of gentamicin in the presence of gastrointestinal lesions. Patients tended to excrete soft stools two or three times a day during oral administration of “GVN”, but without any evidence of malabsorption. Oropharyngeal sterilization was almost completely achieved with the nebulization of gentamicin, vancomycin and amphotericin B in addition to swirling of “GVN” in the mouth before swallowing. In spite of skin disinfection with chlorhexidine, the perineal and axillar regions were the most frequent sites of persistent contamination. Moreover, it was suggested that suppression of both intestinal and oropharyngeal flora would prevent further microbial contamination on the skin surface of patients and, hence, within a protected environment. It was noteworthy that infections due to Herpes simplex virus and Pseudomonas aeruginosa sensitive to gentamicin occurred during successful “GVN” prophylaxis.