RG-13022 significantly inhibited EGF-stimulated autophosphorylation of its receptor in two breast cancer cell lines that have abundant, although not amplified, EGF receptor content (MDA-231 and T47D).RG-13022 suppressed not only EGF-stimulated cancer cell proliferation in vitro but also tumor growth in nude mice. RG-13022 also increased the life span of these tumor-bearing nude mice.