Esterase 1F was isolated from mouse serum and purified by ion-exchange chromatography, isoelectrofocusing, and molecular sieve chromatography. It is considered to be a glycoprotein with an apparent molecular weight of 75 000. The equivalent weight (approximately equal to 77 000 X g/mol) was estimated by titration of the catalytic site with diethyl p-nitrophenyl phosphate. The Michaelis constant Km and the catalytic constant kcat of the enzyme for 4-nitrophenyl hexanoate were determined. Esterase 1F is characterized by its ability to split a wide spectrum of substrates and its relatively low turnover rates towards the substrates tested. It belongs to the isozyme system of carboxylesterase (EC 3.1.1.1) coded for by chromosome 8. Esterase 1F was compared with three other genetically related isozymes, esterase 2, esterase 7 and esterase 9, with respect to some physical and catalytic properties.