Twenty-five patients with metastatic gastrointestinal adenocarcinoma received one to four infusions (400 mg) of murine monoclonal antibody CO17-1A. Eleven patients had mild gastrointestinal symptoms, and one had a transient flushing episode. Two of five who received three weekly infusions had readily reversible anaphylactic reactions at the time of the third infusion (day 15). There were no other toxic effects. One patient had a complete remission and is surviving at greater than 104 weeks, and four had stable disease. The median survival for the whole group was 57 weeks. In general, the antibody infusions were well tolerated but had modest antitumor effects.