In order to elucidate the remaining one minor metabolic product of sulfisomezole in human urine after its ingestion, the human urine was treated with activated carbon, and the adsorbed product was purified through Dowex 50W-X8 and Amberlite IRA-68 columns, paper partition chromatography, and Sephadex G-25 and SE-Sephadex, and the product was obtained as colorless prisms, mp 123-125°. This product was examined for solubility, presence of alcoholic hydroxyl, hydrolysis, and the usual chemical and physical analyses. Nuclear magnetic resonance spectrum indicated the loss of a methyl group from sulfisomezole and appearance of a methylene and hydroxyl. Acetylation of this product with acetic anhydride gave a triacetylated compound. These experimental results indicated the in vivo oxidation of CH3 in sulfisomezole to CH2 and the product was confirmed to be N1-(5-hydroxymethyl-3-isoxazolyl) sulfanilamide.