Tests of 2-(p-aminobenzenesulphonamido)-4, 6-dimethylpyrimidine ('Sulphamezathine') as a prophylactic and curative drug for caecal coccidiosis of chickens have confirmed the previous findings of workers in Great Britain and have shown that the disease can be checked even after intestinal haemorrhage has begun.A related compound, 2-sulphanilamido-4-methylpyrimidine ('Sulfamerazine'), and its soluble sodium salt have been found to have comparable value as curative drugs.This work suggests that these sulphonamides, with partly methylated pyrimidine structures, will prove of value as agents for the control of the disease when used in conjunction with other recognized measures. The relationship between the chemical structure and the coccidiostatic effect requires further investigation.