Immunohistochemical localization of estrogen receptor (ER) using specific monoclonal anti-human estrogen receptor antibody, H222, with an immunoperoxidase technique was performed on fresh frozen tissue derived from 100 endometrial adenocarcinomas. Immunohistochemical evaluation incorporated both intensity and distribution of staining. In all cases, H222 localized in the nucleus of target cells. A significant quantitative relationship was shown between histological score (H-Score) and the biochemical analysis of ER content in tissue homogenates (r = 0.65, P = 0.00001). Excellent sensitivity (92%) and specificity (93%) were observed for the comparison of H-Score to the biochemical assay. Significant ER localization was present in stromal and myometrial elements, component H-Score of which correlated weakly with component H-Scores of malignant epithelial elements. Divergent receptor localization in stromal and myometrial versus malignant epithelial elements suggests that biochemical assays of endometrial carcinoma specimens may not reflect cancer-relevant receptor content. The data presented here suggest that the immunoassay of ER using H222 monoclonal antibody provides additional histochemical information to complement conventional analyses of endometrial adenocarcinomas.