Pirinidazole was found to be the compound from a series of 5-nitroimidazoles with the broadest action against protozoans. Its effect against trichomonads is the most pronounced. The comparison of the effective doses of different 5-nitroimidazoles against Trichomonas fetus demonstrates that the compound Pirinidazole is slightly superior to Tinidazole, distinctly superior to Metronidazole, and very much superior to Nitrimidazine. Some toxicological data of Pirinidazole show that the compound is well tolerated.