The anabolic affects of the IGFs are mediated through the type I IGF receptor (IGFIR), a member of the tyrosine-kinase family of growth factor receptors. Activation of the IGFIR results in its autophosphorylation. This creates motifs containing phosphotyrosine (PY) residues which are recognized by cytosolic proteins involved in IGF signal transduction. A number of such proteins have been characterized, including IRS-1, IRS-2, and Shc, but it is clear that additional proteins remain unidentified. We have previously shown that a yeast two-hybrid system (the interaction trap) can be used to characterize the interaction between the IGFIR and IRS-1 and Shc. We have now used the interaction trap to screen a human fetal brain cDNA library for novel proteins which interact with the IGFIR. This analysis identified three clones which contained src homology 2 (SH2) domains. SH2 domains bind to PY containing motifs and are found in a number of signaling molecules. Each of these clones interacted with the wild type (i.e., tyrosine-kinase active) IGFIR cytoplasmic domain and the insulin receptor cytoplasmic domain but not with a kinase-defective IGFIR mutant. One clone encoded the C-terminal portion of Grb10, a protein recently identified as interacting weakly with the EGF receptor. The other two clones encoded previously unidentified SH2 domains, both of which are most similar to the N-terminal SH2 domain of p85, the regulatory subunit of PI-3 kinase. Additional studies will be necessary to characterize the interactions between these molecules and the IGFIR and to determine if these molecules are involved in IGFIR signal transduction in mammalian cells.