The toxicity to the urothelium of bipotassium ethylene-diaminetetraacetic acid (K2-EDTA) buffered with 0.2 M triethanolamine (TEA) at pH 8 and 8.5 was tested in rats and dogs. Even at a low concentration of 3.125 mM, K2-EDTA is very noxious to the bladder mucosa. This toxicity is not due to the buffer TEA, which is well tolerated. Although buffered K2-EDTA, at pH 8.5 is an excellent chemolytic agent for calcium-containing stones, its clinical use is limited by this toxicity.