A simple single disc technic for determining sulfonamide susceptibility of rapidly growing organisms has been evaluated, and the effect of variations in disc content, inoculum size and medium composition determined. The method was compared with a plate dilution test using ten bacterial species. There was good correlation between inhibition zone sizes and minimum inhibitory concentrations of sulfonamide. Standards of interpretation of zone sizes based on readily obtainable sulfonamide blood levels were proposed. Diffusion rates of the various sulfonamides tested were found to be similar, but considerable differences in their activity against some bacterial species were detected; this was particularly marked with Herellea and Pseudomonas species. Sulfamethoxazole, sulfisoxazole and sulfathiazole had the greatest over-all in-vitro activity of the agents tested. The proposed routine disc diffusion technique was found potentially applicable to all the organisms tested except group-A beta-hemolytic streptococci. It is concluded that single disc sulfonamide sensitivity testing may be of comparable value to antibiotic susceptibility testing if discs of high potency, and media free of sulfonamide inhibitors are employed, and if the criteria for reading and interpreting results are rigidly adhered to.