A further polymorphic rat esterase with broad tissue expression and restricted substrate specificity is described and tentatively called Es-6. Inbred rat strains have either fixed allele Es-6F or fixed allele Es-6S. Es-6 is not linked to the established esterase cluster consisting of the eight esterase loci Es-1, Es-2, Es-3M, Es-4M, Es-4W, Es-5 (= Es-3W), Es-7, and Es-8 in LG V of the rat or to RT1, Gc, c, a, and h. Esterases with apparently identical biochemical and genetical characteristics are Es-17 of the mouse and Es-A4 of humans.