A specific radioimmunoassay (RIA) was established for an acid-stable insulin-like growth factor-binding protein (IGF-BP) isolated from porcine serum. The RIA recognizes a GH-dependent 150 kDa protein in porcine serum; therefore we postulate that the acid-stable IGF-BP is a component of the high molecular weight IGF-BP in porcine serum. The IGF-BP concentration was assayed in porcine serum (normal, 2.44 mg/l; hypophysectomized, 0.83 mg/l; serum from a GH-treated pig, 4.72 mg/l), porcine colostrum (2.55 mg/l), milk (0.91 mg/l), and amniotic (1.82-3.14 mg/l), allantoic (2.94-3.58 mg/l) and follicular (2.28 mg/l) fluids. Serum concentrations of IGF-BP were significantly increased (63%) in pigs chronically injected with porcine GH (pGH) (70 micrograms/kg body weight per day for 17 days). Concentrations of IGF-BP did not change in porcine serum following acute challenges with pGH (10-1000 micrograms/kg body weight) or IGF-I (4 or 8 mg per pig). This is the first report of a specific RIA for the porcine GH-dependent IGF-BP. Our results indicate that this IGF-BP is found in a wide variety of biological fluids and that its concentration appears to be regulated by pGH but not by IGF-I.