In 42 patients with induction treatment of acute myeloblastic and lymphoblastic leukaemia the authors compared efficacy of selective decontamination of the gastrointestinal tract in prevention of infections during neutropenia less than 0.5.10(9)/l in two comparable groups. Twenty-two patients were treated with Ofloxacin (Tarivid, Hoechst Co.), 20 patients with Trimetroprim-Sulfamethoxazol (Biseptol, Polfa Co.). Both groups had concurrently also Ketoconazol in prevention of mycotic infection. The investigation revealed that Tarivid is a suitable alternative drug for selective decontamination, because it delays the onset of acquired infection, as compared with Biseptol, it reduced more efficiently the frequency of Gram-negative colonization and life-threatening Gram-negative sepsis, caused by resistent strains; its tolerance is significantly better. There was no significant difference in the occurrence of febrile days, febrile episodes, the duration of antibiotic treatment, the number of sepsis in two groups. The effect of Tarivid and Biseptol on the Gram-positive microbial flora is inadequate. Subclavian catheter increases in particularly the risk of Gram-positive sepsis in both groups.