Abstract: Compounds comprising selected 2,4-diamino-6-methyl-5-(halophenylazo) pyrimidines which are highly active as folic acid antagonists. The compounds include 2,4-diamino-6-methyl-5-(m-chlorophenylazo) pyrimidine; 2,4-diamino-6-methyl-5-(p-bromophenylazo) pyrimidine; and 2,4-diamino-6-methyl-5-(p-iodophenylazo) pyrimidine.