Monoclonal anti-epidermal growth factor (EGF) receptor antibodies were used as a diagnostic tool for the investigation of the role of the receptor molecule in the transduction of the biological effects mediated by EGF. The specificity of the antibodies was established by immunoprecipitation of the receptor from biosynthetically labeled cells. The previously described (Schreiber, A. B., Lax, I., Yarden, Y., Eshhar, Z., and Schlessinger, J. (1981) Proc. Natl. Acad. Sci U. S. A. 78, 7535-7539) 2F2-IgM antibody binds to or close to the binding site of the growth factor on the receptor molecule; it induces receptor clustering and internalization and triggers early and delayed effects of EGF. A monovalent Fab fragment of the 2G2-IgM antibody stimulates the EGF-receptor sensitive protein kinase, but does not induce receptor clustering and DNA synthesis. When cell-bound 2G2-Fab fragments are cross-linked with anti-mouse Ig antibodies, both receptor clustering and mitogenic activity are restored. A second monoclonal antibody against EGF-receptor denoted TL5-IgG does not interfere with the binding of EGF to the receptor, fails to induce receptor clustering, and does not possess any intrinsic bioactivity. The cross-linking of cell-bound TL5-IgG with anti-mouse Ig antibodies leads to the stimulation of DNA synthesis. It is concluded that the EGF-receptor, when properly triggered, contains all the biochemical attributes necessary for the initiation of biological effects. Receptor clustering, even when mediated via domains distinct from the hormone binding site on the receptor molecule, appears as a necessary and sufficient signal for the induction of DNA synthesis.