The author succeeded in the extraction of a substance from the skeletal muscle of rabbit designated as "muscle cornin", that markedly inhibits of sea urchin eggs. This substance is dialysable and can be separated into three fractions by DEAE-cellulose column. It has been found that fraction Ⅰ has no inhibitory action on mitosis, while fraction Ⅱ and Ⅲ has marked antimitotic effect, its minimum effective dose are 10(-8)g/ml. The fraction Ⅱ is a nucleoprotein with hypoxanthine or xanthine as the ba se and has the maximum absorbance of ultraviolet spectrum at 249mμ. The fraction Ⅲ is a polypeptide that has no specific maximum absorbancy of ultraviolet spectrum. The muscle cornin not only shows a typical protein wave in polarography but it also proves to be positive to ninhydrin and Liebermann reactions, bromphenol blue stain, and precipitation by trichloroacetic acid. However, it is negative to Biuret, xanthoprotein, Sakaguchi and Molish reactions. As for its sugar reaction, it is also positive to Seliwanoff and phloroglucinol reactions, but is negative to other reactions. Therefore, it has chemical properties that are partially different from those of "corneal cornin". The muscle cornin has no effect whatever on catalase activity and oxygen consumption of tissues. In addition, differing from the corneal cornin, it dose not effect the P/O ratio of rat liver mitochondria. However, just as with the corneal cornin, it dose inhibit the incorporation of (32)p into nucleic acids, especially into DNA and r-RNA of regenerating rat liver, and also it inhibits DNA synthesis. But it shows no effect on RNA synthesis. The discussion was also made on the difference in the properties of muscle cornin from those of other inhibitorsof mitosis, extracted from living tissues.