Abstract: Problems concerning the toxicity of tetracycline, 4-epitetracycline, anhydrotetracycline and anhydro-4-epi-tetracycline are discussed in this paper. Among the above preparations, the 4-epi-tetracycline is most toxic to mice. When given intravenously its LD₅₀ to mice is 85.8 mg/kg, as compared with 160.7 mg/kg of tetracycline, 134.2 mg/kg of anhydrotetracycline and 193 mg/kg of anhydro-4-epi-tetracycline. In sub-acute toxicity study anhydro-4-epi-tetracycline exhibited most toxic effects in mice. When given intravenously 1 mg/day, or even 12 mg/day orally, severe diabetic urine and proteinuria were observed, and serious pathologic change of the convoluted tube appeared. Our experiment indicates that anhydro-4-epi-tetracycline may be the chief material responsible for Fanconi syndrome.