The insulin-like growth factor-binding proteins (IGFBPs) in sera of growing pigs were partially characterized with respect to their size, immunological relationships to other known IGFBPs and their regulation by porcine (p) GH. Castrated male pigs (14-16 weeks of age) were treated with either vehicle or pGH (up to 100 micrograms/kg body weight per day) by daily i.m. injection for 7 days. Blood samples were collected by jugular venepuncture at the time of injection. Five IGFBPs of 43, 40, 34, 30 and 26 kDa were identified on ligand blots of porcine sera. A 30 kDa IGFBP, in addition to the 43 and 40 kDa IGFBPs, was immunoprecipitated by antiserum to pIGFBP-3 and found to contain N-linked carbohydrate suggesting that it is a fragment of pIGFBP-3 as has been noted for a 29 kDa N-glycosylated IGFBP in rat sera. The 34 kDa IGFBP in pig sera was precipitated by antisera to rat IGFBP-2 and contained no N-linked carbohydrate. Administration of pGH to normal growing pigs not only increased pIGFBP-3 levels but elicited a dose-dependent suppression of levels of the 34 kDa IGFBP as well. In summary, the Mr pattern of IGFBPs in the sera of growing pigs is similar to that observed in fetal and maternal pig sera and in other species. Furthermore, we report that administration of pGH to normal pigs suppresses the expression of an IGFBP-2-like IGFBP in pig sera while increasing expression of pIGFBP-3.