Of 605 hyperprolactinaemic sera (prolactin > 1000 mU l-1 determined with PRL DELFIA, Wallac) in the routine diagnostic (PRL was measured in 10,737 sera) 26% had an increased amount of PRL with molecular weight (MW) = 150-170 kD (150-170 kD PRL or bigbig PRL). In a series of serum samples from 660 healthy subjects, only one female with hyperprolactinaemia due to increased 150-170 kD PRL was found. The 150-170 kD PRL constituted less than 1% of the total PRL found in 11 patients with prolactinoma (macroprolactinoma, PRL 8800-500,000 mU l-1). When determined with five different commercially available immunometric assays, the PRL values in the sera with large amounts of 150-170 kD were increased in all sera in four of five assays, although they varied widely. The clinical implications of hyperprolactinaemia due to increased 150-170 kD PRL are still uncertain.