Insulin-like growth factor (IGF) action is modulated by six IGF-binding proteins (IGFBP-1 to -6). IGFBP-3 is the main IGFBP in serum and is produced by many cell types, which can modify it post-translationally to yield glycosylation, proteolysis and phosphorylation products. This study investigates the regulation of IGFBP-3 phosphorylation by IGF-I in human neonatal skin fibroblasts. Fibroblasts were incubated with IGF peptides and 32P-orthophosphate for 4 h, and phosphorylated IGFBP-3 (P-IGFBP-3) was immunoprecipitated from the medium and analysed by SDS-PAGE. Media collected from parallel experiments without radioactivity were assayed for immunoreactive IGFBP-3 (I-IGFBP-3). IGF-I (50 ng/ml) increased levels of P-IGFBP-3 and I-IGFBP-3 in conditioned medium to 205 +/- 9% and 198 +/- 10% of control, respectively (n = 5). Stimulation of I-IGFBP-3 was consistent with IGF-mediated release of cell-associated IGFBP-3, since treatment with an IGF-I analogue with reduced affinity for IGFBPs did not increase I-IGFBP-3 levels, whereas treatment with an analogue with reduced affinity for receptor but normal affinity for binding proteins did. In contrast, stimulation of P-IGFBP-3 occurred independently of IGF binding to IGFBP, instead requiring interaction of IGF-I with its receptor. While the functional significance of IGFBP-3 phosphorylation is unclear, we propose that it plays a regulatory role in IGFBP-3 action.