A mouse monoclonal antibody (MoAb), S74 against a human scirrhous gastric carcinoma cell line was developed. The radioiodinated antibody bound preferentially in vivo to xenografted tumors rather than to normal tissue, whereas tumor uptake of normal mouse IgG did not occur. Thus this MoAb was able to selectively localize a human tumor in an animal model, and clinically it may enable immunotherapy against scirrhous carcinoma of the stomach.