Fragment [A-B] of tetanus toxin was highly purified by combination of gel permeation chromatography, adsorption chromatography in an HPLC and immunoadsorption chromatography using anti-Fragment [C] as a ligand. The purified Fragment [A-B] (200 micrograms) elicited a peculiar toxicity, 'hypoactivity' or 'weakness', and killed the mice in ca. 73 hr and 88 hr when it was injected i.v. and i.m., respectively. However, contamination by the whole toxin was not detectable, in the purified fragment preparation, when up to 600 micrograms was tested by the mouse toxicity assay.