The increasing use of intensive cytotoxic chemotherapy for patients with solid tumors enhances the risk of opportunistic infection to levels formerly seen only in patients with acute leukaemia, and prevention of infection is a major concern. A relatively simple regimen of isolation, topical antisepsis, and orally administered non-absorbable antibiltics was stuidied in 18 patients. Sixteen of 21 studies were performed using portable laminar ait flow apparatus and five with isolation only. All patients became severly neutropenic but there were no major infections. Microbiological results showed effective decontamination of the skin, which was maintained without recolonization or acquisition of new organisms. The ears, nose and throat were effectively decontaminated only when the regimen was intensified. Colonization with Pseudomonas aeruginosa, a major pathogen in compromised hosts, did not occur. The propective regimen is less expensive than regimens previously desribed, is acceptable to patients, and requires no modification of existing hospital rooms. It merits further evaluation in patients with common cancers who receive intensive cytotoxic drug therapy.