The causes of hyperprolactinemia are varied, but some cases are classified as "idiopathic" because of unknown causes. We examined whether anti-prolactin (PRL) autoantibodies can cause hyperprolactinemia, especially the asymptomatic type. Serum PRL in four women with anti-PRL autoantibodies and five control patients with prolactinoma was characterized by a sensitive enzyme immunoassay, Nb2-bioassay, gel chromatography, affinity chromatography for immunoglobulin G (IgG), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions, and clearance studies using anesthetized rats. In four women with anti-PRL autoantibodies, serum immunoreactive PRL concentrations were elevated (326 +/- 216 micrograms/L, normal < 30 micrograms/L), and PRL (84 +/- 5.5%) mostly consisted of the large molecular form in which a significant amount of 23 kDa PRL (60.6 +/- 14.7%) was noncovalently bound to IgG. Although three of the four women lacked clinical symptoms of hyperprolactinemia such as amenorrhea and galactorrhea, the IgG-bound PRL was fully bioactive in vitro. It was cleared more slowly from circulation than free PRL. The data suggest that PRL forms a complex with IgG, and this probably results in delayed clearance of PRL and leads to hyperprolactinemia in women with anti-PRL autoantibodies.