Two monoclonal antibodies to progesterone receptor (PR), JZB39 and KD68, were used for the immunocytochemical visualization of PR in different kinds of breast cancer specimens including (1) cryostat sections of tumors frozen at -80 degrees C; (2) paraffin sections of tumors fixed in formalin or in Bouin's fixative for varying periods of time at room temperature or at 4 degrees C; and (3) imprints and cryostat sections prepared from the tissue used for frozen section diagnosis and stored at -80 degrees C after fixation in Zamboni's solution. Sections of conventionally frozen specimens as well as imprints and cryostat sections stored for varying periods of time were stained with the peroxidase-antiperoxidase technique, whereas the avidin-biotin technique was used for paraffin sections. In all types of specimens the PR immunostaining was localized to the nuclei of carcinoma cells and displayed considerable heterogeneity both in intensity and in distribution of positive cells. Close correspondence was found between the different immunohistochemical techniques as well as between immunostaining and steroid-binding assays. PR staining was more frequently positive in well-differentiated than in moderately or poorly differentiated carcinomas, whereas no meaningful correlation was found between PR staining and extent of the disease. Similar results were obtained with the immunostaining of estrogen receptor in the same material using monoclonal antibodies H222 and D75P3 gamma. Thus, by choosing the technique that best suits the type of specimen available, it is possible to obtain valid information on the receptor status of any breast carcinoma, regardless of its size and clinical presentation.