Background Insulin-like growth factors (IGF) regulate fetal growth through their effects on placenta. Their actions are influenced by IGF binding protein-1. Phosphorylated IGFBP-1 (pIGFBP-1) has high affinity for IGF-I and usually inhibits IGF-I activity but during pregnancy, it is de-phosphorylated to generate lower affinity isoforms and consequently, increased IGF bioavailability. Here we investigate the role of placenta in this process. Results Our data show that term human placental explants, but not their conditioned medium, can de-phosphorylate IGFBP-1 through the action of placental alkaline phosphatase (PLAP). Discussion PLAP-mediated de-phosphorylation of IGFBP-1 may provide a mechanism for controlling IGF-I bioavailability and action at the maternal/fetal interface. © 2014 Elsevier Ltd. All rights reserved.