In a controlled randomized study among 48 patients undergoing 75 courses of aggressive antileukemic therapy, it was shown that cotrimoxazole was less effective than penicillin G in preventing septicemia due to viridans streptococci. Both antibiotics were given intravenously. During 35 episodes of chemotherapy in the group of patients on penicillin G only, one patient developed a streptococcal bacteremia; this contrasted with bacteremia and septicemia in seven patients during 40 episodes in the group on cotrimoxazole. In three of these seven patients, septicemia was associated with respiratory failure and it was the cause of death in one. Both aerobic gram-negative rods and streptococci which caused infection despite cotrimoxazole prophylaxis were resistant to cotrimoxazole. Side effects such as hypersensitivity and favorable or unfavorable interaction with the oral selective decontamination regimen were similar for the two drugs, with the exception of colonization with Candida spp, which occurred more often in patients on cotrimoxazole than in patients on penicillin.