After the revelation of the presence of ganglioside GM2 as the major ganglioside in the roe of striped mullet, Mugil cephalus [Li, Hirabayashi, DeGasperi, Yu, Ariga, Koerner & Li (1984) J. Biol. Chem. 259, 8980-8985], we have continued to investigate the catabolism of GM2 in this tissue. We have found that mullet roe contains a specific activator protein which stimulates the hydrolysis of GM2 carried out by the beta-hexosaminidase isolated from the same tissue. This activator has been purified by using conventional procedures including ammonium sulphate fractionation and chromatography on Sepharose 6B, DEAE-Sephadex A-50, octyl-Sepharose and Matrex Gel Blue A columns. This activator protein is also able to stimulate the hydrolysis of GM2 carried out by human beta-hexosaminidase A. Unlike human GM2-activator, the roe activator protein does not stimulate the hydrolysis of GgOse3Cer or GbOse4Cer. The molecular mass (18 kDa) of the roe activator protein was found to be similar to that of human GM2-activator; however, the pI (pH 4.1) was found to be lower than that of human GM2-activator. This is the first report on the presence of a GM2-activator protein in a source other than mammalian tissues.