Amplification and/or overexpression of the erbB-2 gene have been demonstrated in 20-30% of adenocarcinomas of the breast, ovary, lung, and stomach and are associated with aggressive clinical course and poor prognosis. Interference with erbB-2 function by the use of monoclonal antibodies is a promising approach to the treatment of these diseases. In this study we demonstrate that a combination of two anti-erbB-2-specific antibodies inhibited the growth of human gastric tumor cells in vitro. This combination antibody therapy also inhibited the growth of human tumor cell lines growing as xenografts in nude mice and was able to dramatically reduce established tumors. This is the first reported observation of tumor regression induced by anti-erbB-2 monoclonal antibodies. Treatment was not curative in that tumors regrew after 6 weeks. Treatment with either single antibody alone did not inhibit cell growth or tumor formation. Pulse chase and tyrosine kinase activity experiments were used to investigate the activity of the erbB-2 gene product (gp185erbB-2). The formation of complexes by two antibodies was found to interfere with receptor function and mimic some properties of a typical receptor ligand. Selective interference of the erbB-2 receptor by combination antibody therapy may be advantageous for the treatment of human cancers.