AbstractThe insulin‐like growth factor (IGF) signaling system includes the growth factors and their cell surface receptors, along with circulating IGF binding proteins (IGFBPs) that may alter and modulate the action of these neurotrophic hormones. These IGFBPs, along with IGFs and receptors, have been detected in various tissues including the brain. In this study, using polyclonal antibody to human IGFBP‐1 or bovine IGFBP‐2, we found that mouse muscle extracts contain similar‐sized proteins that crossreact with these antibodies on Western immunoblots. After establishing that these antibodies reacted with the homologous murine IGFBPs, we performed immunocytochemistry to demonstrate the localization of IGFBP‐1 at the neuromuscular junction, a model nicotinic, cholinergic synapse, as well as within intramuscular nerves. IGFBP‐2, a distinct macromolecule, is present on the surface of muscle fibers and is not present within synapses or nerves. © 1994 Wiley‐Liss, Inc.