A method is described which enables the quantitative determination of the non-acetylated, non-conjugated fraction and thus the potentially bacteriostatic material of sulfonamide type, occurring in the body fluids during treatment with sulfonamides, by a simple procedure suitable also for serial analyses. In blood plasma, serum or interstitial fluid, the fraction mentioned consists almost exclusively of the unchanged drug which has been administered. In urine it can, in certain cases, comprise in addition a small part of sulfanilamide and/or another closely related and probably also active metabolite, the structure of which – as yet only partially known – is at present being investigated. An analytical process specially adapted for the combination with the former one, but not new in principle, is proposed for determining the total sulfonamide content. The difference of the results obtained by the 2 techniques corresponds to the sum of the non-antibacterially active sulfonamide metabolites contained in the samples.