Antitumor agents including mitomycin C (MMC) and methotrexate (MTX) were coupled to monoclonal antibodies against murine bladder tumor cell line MBT-2 for the purpose of enhancing their drug activity. MMC was conjugated with antibody through two carriers, periodate-oxidized dextran and dithiopyridylated serum albumin, and MTX was conjugated with antibody either directly or through dithiopyridylated serum albumin or poly-L-lysine via an amide bond. These conjugates were assayed for growth inhibitory effect on MBT-2 bladder tumor cells, MCA clone 15 embryo cells and P388 leukemic cells. The cytotoxicity tests demonstrated that drug-antibody immune conjugates were 10 to 100 times more cytotoxic against antibody-reactive MBT-2 cells than nonimmune drug conjugates and showed a similar level of cytotoxicity against antibody-nonreactive cells to that of the nonimmune conjugates. This selective cytotoxicity was also confirmed by competitive inhibition of unconjugated antibody, showing a dependency on antibody binding to the target cell surface antigens. The targeting effect was further assessed by evaluating the suppression of tumor growth subsequent to in vitro treatment of MBT-2 cells with immune conjugates. Half of the mice receiving cells treated with immune conjugates survived more than 40 d after inoculation, while none or one of 7 mice receiving cells treated with unconjugated drug, antibody alone or nonimmune conjugates survived at 40 d after inoculation.