Two kinds of cDNA encoding mouse $Gal{eta}$1,3(4)GlcNAc ${alpha}$2,3-sialyltransferase (mST3Gal III) and $Gal{eta}$1,4(3)GlcNAc ${alpha}$2,3-sialyltransferase (mST3Gal IV) were isolated from mouse brain cDNA library by means of a PCR-based approach. The cDNA sequences included an open reading frame coding for proteins of 374 and 333 amino acids, respectively, and the primary structure of these enzymes suggested a putative domain structure consisting of four regions, like that in other glycosyltransferases. The deduced amino acid sequences of mST3GaI III and IV showed a 98% and 89% identity with rat ST3GaI III and human ST3Gal IV, respectively. Northern analysis indicated that the expression of mST3Gal III mRNA was abundant in heart, liver and adult brain, while that of mST3GaI IV mRNA was detected in all tissues tested except for testis, but the level was the highest in liver. Soluble forms of mST3GaI III and IV transiently expressed in COS cells exhibited enzyme activity toward acceptor substrates containing the terminal either $Gal{eta}$1,3GlcNAc or $Gal{eta}$1,4GlcNAc sequences. The substrate preferences of both enzymes were stronger for tetrasaccharides than for disaccharides.