This work was undertaken to isolate the ATPase accelerating from fur seal muscle hydrolysate. Minced muscle of fur seal was digested with protease and the resulting hydrolysate was treated with ethanol. The obtained peptide mixture was divided by ultrfiltration into two fractions, Fraction A and Fraction B. The ATPase accelerating effect was found to be concentrated in Fraction B, which contained peptides of molecular weight less than 1, 000. By gel filtration on Sephadex G15, Fraction B was divided into ten fractions (G1-10), among which the accelerating effect was significantly high in fractions G6 to G10, the adsorbed fractions as estimated from the Kd values. The effect was pronounced in Fractions G9 and G10. By paper electrophoresis and ion exchange chromatography (DEAE-Sephadex A-25), Fraction G9 was found to be homogeneous.