Ascorbic acid incubated with monoclonal antibodies (22 degrees C, 60 min, pH 6.5) at a molar ratio of 3500:1, reduced 2.7 +/- 0.2% of the available disulfides to sulfhydryl groups that strongly bind 99mTc, and provided greater than 95% labeling efficiency for several IgM, IgG and F(ab')2 antibodies. The colloid formation was consistently less than 3% and the stability of the tracer when challenged with DTPA and cysteine was excellent. The immunospecificity of labeled antibodies as determined by immobilized specific antigen assay was 84 +/- 1% for IgM and 82.6 +/- 1.1% for IgG antibodies. For in vivo evaluation in mice bearing experimental abscesses and tumors, corresponding 125I-labeled antibodies served as controls. The liver uptake was similar (P = 0.76 and P = 0.12) for 99mTc or 125I labeled antinuclear antibody TNT-1 in mice bearing abscesses as well as for 99mTc-TNT-1-F(ab')2 and 125I-TNT-1-F(ab')2 in mice bearing tumors. Higher but statistically insignificant (P = 0.08, 0.18, and 0.73) urinary excretion was noted for 99mTc-antibodies. For corresponding 99mTc- and 125I-labeled antibodies, the abscess to muscle ratios (3.3 +/- 0.5 vs. 3.4 +/- 0.8) and tumor to muscle ratios (10.04 +/- 4.4 vs. 10.54 +/- 3.0) were similar. The high 99mTc-TNT-1-F(ab')2 uptake permitted excellent scintigraphic visualization of tumors whereas the nonspecific 99mTc-HSA did not (tumor/muscle ratio: 2.4 +/- 0.3). This method is simple, reliable, and adaptable to an instant labeling technique.