Streptothricin or streptothricin-like substances show delayed toxicity and this kind of toxicity is considered to be due to a kidney damage caused by these substances. If a kidney is damaged and excretion of the toxic substance is delayed, then the toxic substance accumulates and causes stronger kidney damage, and finally the animals are led to death. In such a case, when a minimal lethal dose is injected, the animals are expected to die a few or several days after the injection. A substance which reduces toxicity of streptothricin may reduce toxicity of other water-soluble basic substances, because water-soluble basic substances having free amino group may be detoxicated by the same mechanism. The present studies were made for a purpose of finding a substance reducing ototoxicity of useful antibiotics such as streptomycin and kanamycin.

A substance having amino group may be detoxicated through formation of glucuronide, then it is interesting to examine the effect of those substances which are expected to stimulate the detoxicating system of basic substances. From another view point, a basic substance which has low toxicity to kidney may compete with streptothricin in the binding sites in kidney cells and may expel streptothricin from the cells. From these two kinds of aspects, the authors examined influnces of pyrimidine derivatives for the effect of reducing toxicity of streptothricin, and a preliminary result 1s presented in this paper.