Claudiximab is a monoclonal antibody specific for gastric and gastroesophageal adenocarcinomas. Preclinically, claudiximab was shown to inhibit tumor growth and to kill cancer cells by indirect (complement-dependent cytoxicity, antibody-dependent cellular cytotoxicity) and direct mechanisms (antiproliferative and proapoptotic effects). The aim of this phase I study is to establish safety, toxicity and maximal tolerable dose of a single infusion of claudiximab in patients suffering from relapsing, advanced gastroesophageal and gastric adenocarcinoma