Recent experiments have demonstrated that egasyn not only sequesters beta-glucuronidase in microsomes by forming high molecular weight complexes with beta-glucuronidase, but also has carboxyl esterase activity. We have found several new phenotypes of egasyn-esterase after electrophoresis and isoelectric focusing of liver homogenates and purified egasyn of inbred and wild mouse strains. Several phenotypes corresponded in relative mobility and relative isoelectric point among inbred strains to that recently reported for esterase-22 by Eisenhardt and von Deimling [(1982). Comp. Biochem. Physiol. 73B:719]. This genetic evidence, plus a wide variety of comparative biochemical and physiological data, indicates that egasyn is identical to esterase-22. Both parental types of egasyn isozymes are expressed in heterozygous F1 progeny, suggesting that alterations in the egasyn structural gene are responsible for the altered isoelectric points. Also, egasyn is a monomer since no new esterase bands appear in F1 progeny. The variants in isoelectric point of egasyn map at or near the egasyn (Eg) gene within the esterases of cluster 1 near Es-9 on chromosome 8.