We conjugated the chemotherapy agent daunorubicin to the anti-T-cell monoclonal antibody T101 using an active ester intermediate of the acid-labile linker cis-aconitate anhydride. By converting carbohydrate hydroxyl groups on the antibody to amines prior to conjugation, average drug to antibody ratios of 25:1 were achieved with retention of cytotoxicity and only minimal loss of immunoreactivity. The pH sensitivity of the linkage was confirmed. The preparation was cytotoxic for antigen-bearing cells but not antigen-negative cells, even up to 48-h incubation in vitro. Specific cytotoxicity was apparently mediated through the endocytosis of the intact T101 immunoconjugate and the release of the active drug in the lysosomal compartment. Athymic mice bearing human tumor xenografts who received a single injection of the immunoconjugate had less tumor growth and more tumor regressions than animals receiving antibody alone, drug alone, or a mixture of drug plus antibody. This approach appears promising for further investigation.