In this experiment, the Dextran-T40 (Dex-T40) served as intermidiate carrier, conjugated SM6 monoclonal antiidiotypic antibody (anti-Id-McAb) against B lymphocytic leukemia cells and chemotherapeutical drug daunorubicin (DNR) to form SM6: Dex: DNR immunoconjugate. The activity of anti-Id-McAb and DNR of the immunoconjugate and their extent of the substitution were measured, and the antitumor activity of SM6: Dex: DNR was assayed in vitro. The results demonstrated that using Dex-T40 to prepare immunoconjugate could get a higher extent of substitution of McAb and DNR and maintain the activity of McAb and drug well. The results of cytotoxic assay in vitro suggested that the immunoconjugate SM6: Dex: DNR possess a specific cytotoxic effect to target tumor cells.