A direct method for 99mTc-labeling monoclonal antibodies (MAb) has been evaluated for labeling intact and F(ab′)2 1A3, an anticolorectal carcinoma MAb. The method employs ascorbic acid to reduce the MAbs. By altering the reaction conditions for 99mTc-1A3, a maximum radiolabeling yield of 48% was obtained with an immunoreactivity (IR) value of 87%; and for 99mTc-1A3-F(ab′)2, a yield of 49% and an IR value of 70% was obtained. Biodistribution of 99mTc-labeled 1A3 MAbs was performed in a Golden Syrian hamster model and compared to 125I-labeled 1A3 MAbs. Tumor uptake (%ID/g) was significantly better for the intact 125I-1A3 at 24 h post-injection compared to the intact 99mTc-1A3. For 99mTc-1A3-F(ab′)2, %ID/g tumor was low, and did not increase over 24 h. High %ID/g kidney persisted at 24 h for both 99mTc-labeled intact and F(ab′)2 1A3. Serum stability was performed in Sprague-Dawley rats for the 99mTc-labeled 1A3 MAbs, and compared to 125I-labeled 1A3 MAbs, which showed intact 99mTc-1A3 cleared similarly to 125I-1A3, and 99mTc-1A3-F(ab′)2 cleared more rapidly than 125I-1A3-F(ab′)2 indicating instability of the 99mTc-labeled 1A3-F(ab′)2.