The author has studied, using the Klebsiella phage-host bacteria system, on the bactericidal effect of the mixture of sulfa-drug(SD) and phage. The following results were obtained. The bactericidal activity of the mixture of SD and phage was stronger than that of SD or phage alone. Under the SD of about 10.0mg/ml, more than about 10-6/ml of phage must be added to SD for expecting the stronger bactericidal activity of SD plus phage than that of SD alone. In SD, about 10.0mg/ml, alone, the number of surviving bacteria was a little increased for 20 hours-incubation at 37`C. However, if the phage of about 10-6/ml is added, the bacteria were much killed. The host bacteria were a little killed by the SD of 10.0mg/ml for 2 hours at 37`C. The SD did not show any influence on the phage activity and phage absorption to host cell. The phage growh, however, in host cell was considerably inhibited by SD.