Previous studies have suggested that the binding of mouse glucuronidase to endoplasmic reticulum membrane is stabilized by the membrane protein egasyn. Using a radioimmunoassay for egasyn, we have now examined the inheritance of egasyn levels in mice. Mice of the ibred strain C57BL/6J, which have normal levels of microsomal glucuronidase, contained 56±10 g egasyn per gram of liver. Mice of the inbred strain YBR, which carry the Eg ⁰ mutation resulting in the absence of microsomal glucuronidase, did not contain detectable levels of egasyn. The F₁ progeny of these two strains contained intermediate levels of egasyn, 25±4 g egasyn per gram of liver. Progeny from the backcross of these F₁ animals to YBR were distributed equally into two discrete phenotypic classes. One class lacked both egasyn and microsomal glucuronidase, while the other class contained 25±3 g egasyn per gram of liver and contained normal levels of microsomal glucuronidase. Thus egasyn levels are determined by the Eg locus and show additive inheritance. These results suggest that the Eg gene codes for egasyn and that it is the inability to produce egasyn that results in a deficiency of microsomal glucuronidase in the Eg ⁰ mutant.