Mouse egasyn cDNA was inserted into expression vector pCDpoly and transfected into mammalian cell lines. Transfected human HepG2 cells, monkey COS-1 cells, and mouse L cells expressed egasyn-esterase catalytic activity. Within COS-1 cells, egasyn was localized to the endoplasmic reticulum. Although individual cells produced large amounts of egasyn, no secretion was observed. No beta-glucuronidase-egasyn complexes were formed in transfected HepG2 or COS-1 cells. However, these complexes were readily detected in transfected L cells. Although the signal for retention of egasyn in the endoplasmic reticulum appears to be species independent, the signal for association with beta-glucuronidase is species restricted.