The type-specific main (TM) antigen was extracted from the leptospiral cells of strain Kyoto of the Hebdomadis group by 90% phenol and was purified mainly by ethanol precipitation. The TM antigen was specific, forming one precipitin band by immunodiffusion and reacting by complement fixation test with anti-strain Kyoto serum; so far as has been tested, it does not appear to react with antisera of other serotypes. The sedimentation velocity of the TM antigen was 216S. Morphologically the TM antigen was filamentous in shape with a uniform width of 25 to 30 nm and various lengths from 50 nm to 1 mum. The infrared absorption pattern of the TM antigen generally resembled that of gram-negative bacterial endotoxin, and this fact suggests that the antigen is composed mainly of lipopolysaccharides. Galactose, rhamnose, xylose, arabinose, glucosamine, 13 kinds of acidic and neutral amino acids, lipids, and phosphorus were found in the TM antigen.