We have evaluated five compounds, stannous chloride (SnCl2), 2-mercaptoethanol (2-ME), dithiothreitol (DTT), dithioerythritol (DTE), and ascorbic acid (AA) to reduce monoclonal antibody MoAb (disulfide groups and compared their efficacy for labeling MoAbs with 99mTc. The reduction of 99mTc with dithionite at pH 11 was nearly quantitative. The use of AA, at a molar ratio of 3500:1, for three IgG and three IgM antibodies examined, gave a labeling efficiency greater than 95%. Hence no purification was needed. The immunospecificity of AA preparations determined by specific antigen assay was 84 +/- 1% for an IgM and 82.6 +/- 1.1% for an IgG, highest among all agents tested. The stability of the tracer was evaluated by challenging the product with such 99mTc avid agents as cysteine, DTPA, and human serum albumin. By HPLC analysis, no 99mTc was transchelated using chelating agent to protein molar ratios as high as 500:1. In two separate groups of five mice each, the liver uptake at 4 h post injection averaged 6.8 +/- 2.9% per gram for 125I-TNT-1 (IgG) and 6 +/- 5.1% per gram for the same MoAb labeled with 99mTc using AA. The AA technique promises to label antibodies with 99mTc and perhaps with 186Re, by a simple "kit" procedure.