Trichomonacidal activity in vitro of 2 newly synthesized derivatives of benzoizothiazolinon (BIT) in comparison with metronidazole was determined by calculating lethal concentration for 50% trichomonal population (C50). Experiments were carried out on 2 strains of T. vaginalis and 2 strains of T. anseris. Both preparations showed considerably higher trichomonacidal activity than metronidazole. Mean values of trichomonacidal activity of N-morpholinomethyl-1,2-benzoizothiazolinon-3 for T. vaginalis and T. anseris amounted to 885 and 1997 respectively, and were 6 and 8 times higher than those of hydrochloride of N-piperydynomethyloamide of (5-chloro-1,2-benzoizothiazolinon-3ylo-2)-acetic acid. The lowest trichomonacidal activity of hydrochloride of N-piperydynomethylamide of (5-chloro-1,2-benzoizotiazolinon-3ylo-2)-acetic acid was 10-fold higher than that of N-morpholinomethyl-1,2-benzoizothiazolinon-3 and several hundred times lower than metronidazole concentration.