Two different murine monoclonal antibody Fab fragments specific for p97, a melanoma-associated antigen, were labeled with I-131 at high activity levels without excessive chemical damage. Up to 20 mg of Fab were labeled with up to 300 mCi of I-131 using the chloramine-T method and large working volumes at room temperature. As much as 90% of the initial activity was recovered as labeled product. The labeled Fabs varied in their sensitivity to radioiodination damage, as measured by an in vitro cell-binding assay. Radioiodination was performed safely using a remote iodination apparatus. The final product was of radiopharmaceutical quality suitable for clinical diagnosis and experimental radiotherapy in humans.