The antagonistic substances used in our experiments showed various relationships to the pathogenic protozoa Trichomonas foetus. Theseinacid and L-2-Methyl-2-Aminoaethenole demonstrated comparable inhibiting influences on the Trichomonas population. Puriniribosid, 6-Mercaptopurine, 4-Mercaptopteridine and Decobald-Cobamide exercised no recognisable influence on the Trichomonas foetus.