This study reports the presence of a large molecular sized PRL as the major form of circulating immunoactive PRL in five women with idiopathic hyperprolactinemia and normal menses. Gel filtration patterns of serum from these patients revealed 98-100% predominance of a 150,000- to 160,000-dalton PRL in contrast to the predominance of the 22,000-dalton species in other hyperprolactinemic patients. This 150,000- to 160,000-dalton PRL was immunologically similar to the 22,000-dalton PRL, and its size on gel filtration was not altered using denaturing conditions. With reduction of disulfide bonds, there was a shift of the peak I PRL to smaller mol wt peptides. In addition, studies of one woman with idiopathic hyperprolactinemia and normal menses revealed preservation of 98% peak I predominance during physiological and pharmacological perturbations of PRL secretion. Finally, assay of the bioactivity of the large molecular sized PRL in the Nb2 rat lymphoma line revealed diminished activity compared to the 22,000-dalton species. This latter finding may help explain the maintenance of normal menses and relative lack of clinical signs in patients with this form of hyperprolactinemia.