125I-labeled monoclonal antibody 19-24 (mouse isotype IgG1) was evaluated for its potential usefulness in the clinical radioimmunodetection of sarcoma. The antibody reacts with a cell surface antigen preferentially expressed in many human soft tissue and bone sarcomas. Chromatographic and electrophoretic analyses indicated that the labeled preparation was relatively pure. Binding studies in vitro demonstrated that specificity for antigen was retained after iodination and indicated that the labeled antibody possessed an immunoreactivity in excess of 90% and a binding constant of 8.1 X 10(9) M-1. When administered to athymic NCr-nu/nu mice bearing 1-cm diameter human fibrosarcoma HT-1080 xenografts, the labeled antibody preferentially localized in tumor deposits. Maximum tumor-to-blood radioactivity ratios (2.2-3.4) were obtained 7 days after antibody injection. Specificity of the localization was confirmed with a control mouse IgG1 antibody and by using a nonreactive xenograft. Distinct tumor images were obtained by gamma camera without the use of subtraction techniques, demonstrating the possible clinical utility of the labeled antibody.