A comparison of mandelamine, streptomycin and sulfathia-zole against 8 stock laboratory strains and 9 strains of organisms recently isolated from urinary tract infections indicates that their activities are not widely different. In contrast to the results obtained with streptomycin and sulfathiazole, resistance to mandelamine either did not appear or, if it did, it appeared slowly and to a questionable degree in 3 organisms, while in 3 other organisms, resistance did not appear at all. Organisms rendered resistant to sulfathiazole or streptomycin remain susceptible to mandelamine.