include the sulfonamides, 2,4-diaminopyrimidines and ethopabate. These compounds are structural antagonists of folic acid or of para-aminobenzoic acid (PABA), which is a precursor of folic acid. (The host does not synthesize folic acid and has no requirement for PABA.) Coccidia rapidly synthesize nucleic acids, accounting for activity of PABA antagonists. Although resistance to antifolate compounds is widespread, they are commonly used for water treatment when clinical signs are already evident. Diaveridine, ormetoprim, and pyrimethamine are active against the protozoan enzyme dihydrofolate reductase. They have synergistic activity with sulfonamides and often are used in mixtures with these compounds.