A series of 1-substituted dinitropyrroles was tested for trichomonacidal activity. The shorter the substituent the greater was the in vitro activity. No relationship was found between in vivo activity and substituent length. A comparison of the trichomonacidal activities of l-[(2,4-dinitropyrryl)-ethyl]-2-methyl-5-nitroimidazole (ORF-2245) and l-(2-hydroxymethyl)-2-methyl-5-nitroimidazole (8823 R.P.) was made. ORF-2245 exhibited activity equal to or greater than 8823 R.P. except in the treatment of established infections where its limited activity was attributed to poor absorption from the gut. Absorption, however, was apparently sufficient to combat or prevent the low-grade infections.HE trichomonacidal activity of l-(2-hydroxyethyl)-2-methyl-5-