The nephrotoxicity of rats caused by dibekacin (3'',4''-dideoxykanamycin B) or kanamycin, with or without dextran was effectively reduced by D-glucaro-.delta.-lactam K salt, as evidenced by lower levels of blood urea N and kidney edema rate, better excretion of antibiotics and less morphological damage. Protection was dosage related, and potentiated with increasing doses, but only when the 2 drugs were given simultaneously. Among 3 alkali-metal salts examined, the K salt was almost equal to the Li salt, but surpasses the Na salt in effectiveness. Inorganic salts, in particular potassium chloride, were found effective for the protection of normal rats, but their effect decreased for the dehydrated rats, especially in the presence of dextran.