Murine monoclonal antibody (MoAb) 17-1A, which reacts with an adenocarcinoma-associated antigen, has recently been utilized in a phase I clinical trial for patients with gastrointestinal tract cancers in Japan. In order to analyze anti-idiotypic (Id) antibodies to murine MoAb 17-1A in the sera of these cancer patients, we established a simple and specific assay. In a modified sandwich assay, normal mouse sera were utilized for neutralization of rat antimouse immunoglobulin (Ig) antibodies and reduction of their nonspecific effects. When the modified sandwich assay was applied to the sera of patients who had been treated with MoAb 17-1A, anti-Id antibodies were induced in 53% of 13 cancer patients with gastrointestinal cancers at 3-4 weeks after infusion of MoAb.