Histochemistry and electron microscopy were used to examine hyaline material within the endometrium of mice continuously fed diethylstilbestrol (DES). Virgin female mice were fed diets containing 0, 320 or 640 ppb DES from 4 weeks of age. All animals reported on in this study were sacrificed when moribund and ranged from 622 to 762 days of age. Irregular deposits of hyaline material were seen throughout the connective tissue stroma with frequent accumulations seen immediately beneath the surface epithelial membrane and around blood vessels. The material was characterized by a paucity of cells and fibrous material but exhibited an eosinophilic reaction and was PAS-positive. Histochemistry indicated collagen was present even though electron microscopy revealed a homogeneous, amorphous structure virtually devoid of a fibrous component. Numerous plasma cells were seen in close proximity to macrophages and lymphocytes. This material was probably a product of collagen degradation and a subsequent immune response elicited in the endometrium.