This report investigates the application of monoclonal antibody A7 and its drug conjugate in locally controlling colorectal cancer. The experimental protocol consisted of local retention, lymphatic delivery, normal organ distribution, systemic toxicity, and tumoricidal effects. When 125I-labeled monoclonal antibody (Mab) A7 was injected into the pelvis and the thigh of Balb/c mice, a high local retention unrelated to antigen-antibody interaction was observed at the injected site for 24 h after injection. An analysis of local retension properties related to antigen-antibody interaction, conducted by intratumorally or peritumorally injecting 125I-Mab A7 into the tumor-bearing athymic nude mice, revealed a significantly higher tumor localization of Mab A7 in comparison to i.v. injection. 125I-Mab A7 accumulated to a great extent in the ipsilateral regional lymph node but not in the contralateral regional lymph node. Normal organ accumulation of Mab A7 was lower in the locally injected group than in the i.v. injected group. Intratumoral injection of Mab A7-neocarzinostatin (A7-NCS) led to the complete remission of established tumor in 5 of 6 antigen-positive xenograft-bearing mice but exhibited a complete remission in only 1 of 6 antigen-negative xenograft-bearing mice. A single local injection of A7-NCS inhibited tumor development in 12 of 16 and 5 of 15 antigen-positive tumor-bearing mice and antigen-negative tumor-bearing mice, respectively, whereas neither a systemic injection of A7-NCS and NCS nor a local injection of NCS and saline had a notable inhibitory effect on tumor development. Systemic toxicity of NCS was markedly reduced when it was locally administered in the antibody-conjugated form. These findings indicate that local injection of immunoconjugate is a promising new field for controlling the local recurrence of colorectal cancer.