ABSTRACT There are two types of insulin-like growth factor (IGF) receptors. The type I receptor generally binds IGF-I more tightly than IGF-II and also interacts weakly with insulin. The type II receptor prefers IGF-II over IGF-I and does not recognize insulin. The type I receptor is made up of an alpha binding subunit (Mr 130000) and a beta subunit (Mr 95 000) probably organized as a heterotetramer (α2β2). The type II receptor consists of a single binding unit (Mr 250000). IGF stimulates phosphorylation of the beta subunit of the type I receptor in whole cells and solubilized receptor preparations. Tyrosine kinase activity is associated with the type I receptor, resulting in autophosphorylation of the beta subunit and phosphorylation of exogenous substrates. In contrast, phosphorylation of the type II receptor in whole cells is less IGF-dependent, solubilized receptor preparations are not phosphorylated, and purified type II receptors do not exhibit tyrosine kinase activity toward the artificial substrate poly(Glu,Tyr)4:1. There are many similarities between the type I IGF receptor and the insulin receptor; however, different ligand-binding properties, subtle differences in the size of alpha and beta subunits, and immunoreactivity toward anti-receptor antibodies allow us to distinguish between these two receptors. The presence of both IGF receptors as well as insulin receptors on most cells and cross-reactivity of ligands for binding to these receptors present difficulties in assigning a particular biological response to a specific receptor. The type I receptor is down-regulated by ligand while in several cell types the type II receptor is rapidly up-regulated by insulin; the mechanism of up-regulation appears to be a translocation of type II receptors to the cell surface. There are two classes of serum binding proteins for IGF, a Mr 150 000 species found in adult blood and a Mr 40 000 species, which predominates in foetal blood. Like the type II receptor, IGF binding proteins do not bind insulin. The binding site on the type II receptor can be distinguished from the binding protein sites by a hybrid molecule Ainsulin-BIDF-I, which recognizes the binding protein but not the type II receptor. Binding proteins produced by cells in culture may cause confusion in the interpretation of experiments that are designed to study the binding of radiolabelled IGF to cell surface receptors in monolayer culture.