A number of studies have been published to evaluate the inmunocytochemical assay for ER using monoclonal antibodies. Histoscores so far used consider two variables: the number of cells and the intensity of the reaction. There are however indications that only the proportion of stained cells are important for assessment and show a direct correlation with quantitative data. We studied 77 breast invasive adenocarcinomas stained with the Abbott ERICA kit and used a simple scale of 0 to 4 for the estimation of ER. Tissues were snap frozen in liquid N and immunostained with the specific antibody and peroxidase. Immunostaining was estimated in a simple observational scale from 0-4+ where 0 = no staining or few scattered positive cells; 1+ up to 25%; 2+ up to 50%; 3+ up to 75% and 4+ more than 75% of stained malignant cells. Counts were performed in at least 100 malignant cells in various microscopic fields. Staining was always nuclear and a considerable heterogeneity in the number of cells and the intensity of the reaction was observed. Grading specimens from 0-4+ was found simple and reproducible. In 45% there was no immunostaining and 54% were positive for ER. In patients greater than 50 years of age 67% were positive; in patients less than 50 years of age only 33% had ER. This procedure has many advantages for clinical use: it is simple, it does not require sophisticated equipment, it is reproducible and can be performed in small tissue fragments, such as needle aspiration material, as well as in cytological smears.