IC₅₀ values were determined by incubation of the tumor cell lines for 96 hours to the hBU12-vcMMAE conjugate or MMAE, as indicated. Cell lines, in which less than 50% cytotoxicity was achieved at concentrations greater than or equal to 300 nM, were considered nonresponsive. Both unconjugated hBU12 and a control, nonbinding vcMMAE conjugate exhibited negligible antitumor activities against these cell lines. All values obtained for hBU12-vcMMAE are significantly different from control-vcMMAE compounds tested in the same assay (Student t test, data not shown). Jurkat cells were used as a CD19⁻ control cell line. The absence of activity of hBU12-vcMMAE on these control cells suggested that the antitumor activity is immunologically specific. Data shown represent the mean plus or minus SD of three replicates from 2 independent experiments.