Steroid hormone receptors are elusive, labile regulatory proteins which communicate the action of the sex hormones, estrogens and progestins, in target organs such as the breast and uterus. Currently, results from analysis of these receptors are used as predictive indices of therapeutic response to endocrine manipulation and as prognostic indicators of the clinical course of patients with either breast or endometrial carcinoma. The commonly accepted clinical methods of estimating these proteins in tumor biopsies are the multipoint titration assay and sucrose density gradient centrifugation, each of which utilizes radiochemically labeled ligands to measure specific binding capacity. In 1977, we established a reference laboratory to monitor the performance of these assays and insure uniformity of methods of analyses and format of reporting clinical data. Sucrose density gradient centrifugation suggested that there were, at least, large molecular weight forms (8S) and lower molecular weight species (4S) based on properties of size and shape. The origin and physiological significance of these multiple forms of receptors have been major foci of our investigations. Although certain of these components may represent distinct physiologic species, some may arise due to proteolytic cleavage. Although numerous attempts have been made to purify these receptor proteins, little conclusive evidence has been reported regarding their native state in breast and endometrium. To circumvent the problem of prolonged manipulation of receptor preparations, our laboratory developed the use of high-performance liquid chromatography in size-exclusion ion-exchange, chromatofocusing and hydrophobic interaction modes for the rapid separation of receptor isoforms. We define receptor isoforms as the protein components in a hormone target organ which exhibit a high ligand binding affinity and specificity for a single class of steroid hormones (e.g. progestins) which may be identified based upon their characteristics of size, shape, surface ionic and hydrophobic properties. Using this procedure we have demonstrated that these steroid receptor proteins exhibit molecular heterogeneity (polymorphism). Results from these studies suggest that the levels of receptor organization and isoform interrelationships are more complicated than thought previously. Recently, our laboratory demonstrated that purified estrogen receptor is associated with protein and phospholipid kinase activities. This has been accomplished by micropurification of the receptor using either affinity chromatography or precipitation of the receptor with monoclonal antibodies immobilized on polystyrene beads.(ABSTRACT TRUNCATED AT 400 WORDS)