Epidermal growth factor (EGF) receptor from A-431 cells was purified by affinity chromatography with monoclonal anti-receptor antibodies. The purified radiolabeled receptor was incubated with EGF and then analyzed by gel electrophoresis under nondenaturing conditions. In these gels, the EGF receptor migrates in two forms: a fast-migrating (low) form and an EGF-induced slow-migrating (high) form. On the basis of the various control and calibration experiments described, it is concluded that the low form represents the monomeric 170-kilodalton EGF receptor and the high form represents an EGF receptor dimer. The binding of EGF causes a rapid, temperature-sensitive dimerization of the EGF receptor. Receptor dimerization is fully reversible and involves saturable, noncovalent interactions that are stable at neutral pH and in nonionic detergents. Both the monomeric and dimeric forms of the receptor bind EGF and undergo self-phosphorylation. The dimeric form of the receptor may possess higher ligand binding affinity, and it seems to be phosphorylated earlier than the monomeric form following the addition of EGF and [gamma-32P]ATP. On the basis of these results, it is concluded that receptor oligomerization is an intrinsic property of the occupied EGF receptor and that it may play a role in the activation of the kinase function and the subsequent transmembrane signaling process.