COMBINED therapy with two or more antibacterial drugs may be useful in the treatment of an individual case, and in discouraging the development of resistance1. A formula was previously presented which could be used in evaluating the combined effect of two drugs2. An organism, highly resistant to penicillin alone and streptomycin alone, is often not affected by a combination of both, when they are combined at their attainable blood levels. In this instance addition of a third antibiotic may yield an inhibitory combination at concentrations of their respective blood levels.