Plasma hPRL consists of a complex mixture of molecular forms. The monomeric form, derived from the pituitary, is the main form. Others (dimeric or oligomeric) can form de novo in plasma. Recently, BBPRL (big big PRL) has been identified in some cases as an antiPRL autoantibody, but these data require further investigation. The PRL forms are differently recognized by immunoassays (IRMA and RIA) and are a source of inter-assay discrepancy.