Study of molecular heterogeneity of immunoreactive prolactin in patients with macro- and microprolactinomas and idiopathic hyperprolactinemia has shown heterogeneity of the total blood immunoreactive prolactin pool in all the examined patients. This pool included three basic forms with molecular masses about 23, 50, and over 100 kD (23K-, 50K-, 100K-prolactin), whose ratios essentially differed in individual patients. Physiologically the most active monomeric 23K form of prolactin predominated in the blood of patients with hyperprolactinemia due to hypophyseal micro- and macroadenomas, parallelled by manifest signs of galactorrhea and hypogonadism; the content of this form may reach 95% of the total immunoreactive hormone. Patients with hyperprolactinemia of obscure origin present with quite the contrary ratio of prolactin immunoreactive forms with the predominance of high-molecular 100K form of the hormone, whose share may reach 80-90%. Such cases of hyperprolactinemia may be associated with the absence of clear-cut clinical manifestations, including such as galactorrhea and menstrual cycle disorders, and with inefficacy of traditional therapy. Such cases were tentatively classified as 'the high-molecular prolactin syndrome' and need further research.