S-alkylthiuroniumhalides according to formula I (table 1) have been tested under standardized conditions with respect to their antimicrobial activity against bacteria, yeasts and fungi. In table 2 the bacteriostatic and fungistatic efficiency and in table 3 the microbicidal efficiency of 8 different S-alkylthiuronium compounds are compared with the efficiency of N,N-dimethyl-coconutoilalkyl-benzyl-ammonium-chloride. The germ inhibitory effect of the quaternary ammonium compound has been superior to the S-alkylthiuronium salts in almost all respects. The germicidal effect of the S-alkylthiuroniumhalides depends on the alkyl chain length bound to the S-atom and gets the optimum with S-dodecylthiuronium-chloride. Already a 0.0005% concentration is sufficient to kill all test germs within 30 minutes. S-dodecylthiuroniumchloride proved to be just as effective as the known well effective quaternary ammonium compounds. It seems to be possible to increase the microbicidal efficiency by additional alkylating - specially methylating - of both nitrogen atoms.