More than 100 1-methyl-5-nitroimidazoles substituted in the 2-position via an oxymethyl group were synthesized and their structure-activity relationship toward various protozoa was investigated. Among the derivatives substituted with an aromatic radical there are most of the compounds which are highly effective against trichomonads; 9 preparations are superior to tinidazole and 29 are superior to metronidazole in mice infected with Trichomonas fetus, and 9 compounds exhibit a better effect than metronidazole in golden hamsters intrahepatically infected with Entamoeba histolytica. In the same series one dialkylamino-acetamide derivative shows excellent trypanocidal activity in the NMRI mouse, but this effect is limited to Trypanosoma brucei; 12 preparations developed a trypanocidal effect only after relatively high doses; their range of efficacy included Trypanosoma cruzi, among others, after repeated treatment. Of the carboxyl acid, carbamic acid and sulphonic acid esters synthesized only the already known group of carbamic acid esters possesses a pronounced antiprotozoal effect. Among the preparations substituted with a heterocyclic radical some of the pyridine derivatives proved to have distinct trichomonacidal activity. The influence of the type of substitution and the stability of the C-X bond in 2-substituted 5-nitroimidazoles of all compounds synthesized so far (I--III, 4th report) are discussed in 2 tables.