Cysteamine [2-mercaptoethylamine (CySH)] displays a variety of neuroendocrine effects, the most potent being the depletion of immunoassayable tissue PRL. The present study used bovine adenohypophysial secretory granules to characterize this inhibition of measurable hormone (assayability). CySH decreased assayability in a dose-dependent manner at pH 7.4, with 50% inhibition observed close to 2 mM. Maximal inhibition was found between pH 6.0 and 6.5, whereas diminished (or no) inhibition occurred under alkaline conditions, depending on the buffer. In contrast, reduced glutathione (without CySH) increased assayability at pH 8-8.5, had little potency near neutrality, and inhibited assayability under acidic conditions. Electrophoretic studies under nondenaturing conditions demonstrated that CySH exposure of standard PRL resulted in additional charged species. With granules, CySH markedly decreased the staining of the major PRL band, no new bands were evident, and this effect was abolished by glutathione. Sodium dodecyl sulfate-electrophoretic patterns indicated that CySH resulted in higher apparent mol wt species of granule and standard PRL. This effect was nullified by mercaptoethanol. Never was there evidence for species smaller than monomeric. Depletion of PRL might involve interference with the conversion from oligomeric storage PRL to assayable PRL; 44-fold increases in PRL oligomer immunoactivity after alkali and thiol treatment were reduced to 6-fold increases when CySH was present. Reactions involved in production of assayable hormone appear relevant to secretion, since CySH also inhibited PRL release from granules, with maximal inhibition occurring under acidic conditions. Thus, CySH may be useful in investigating the physicochemical properties of tissue PRL and may also represent an approach to treatment of hyperprolactinemic states.