AbstractWe report an immunohistochemical method for the assay of estrogen and progesterone receptors (ER/PR) in zinc-sulfate formalin fixed, paraffin embedded tissues of breast carcinomas with Abbott's monoclonal antibodies to ER protein (H222) and PRprotein (JZB39). Small portions of tumor were placed in zinc sulfate formalin, which was then used either as a primary- or a postfixative. Sections were pretreated briefly with trypsin and pronase, and the combination of a supersensitive streptavidin detection system with dilution of the primary monoclonal antibodies yielded an immunoreaction with high nuclear signal and low background. In those instances when only small amounts of breast cancer are available, as is increasingly common, this technique may prove invaluable. (The J Histotechnol16:51, 1993)