Dose-dependent cytotoxicity of the isatin derivatives 1-5 is maintained in the vinblastine resistant U937 (U937VbR) subline. Cells were incubated with increasing concentrations of either (A) 5,7-dibromoisatin (1), (B) 5,7-dibromo-N-(p-hydroxymethylbenzyl)isatin (2), (C) 5,7-dibromo-N-(p-phenylbenzyl)isatin (3), (D) 5,7-dibromo-N-(p-cinnamyl)isatin (4), (E) 5,7-dibromo-N-(naphthalen-1-ylmethyl)isatin (5) or (F) vinblastine (as a control). The relative sensitivity of the U937 (black) and U937VbR (red) cell lines to the drugs was determined by MTS assay after 48 h incubation. Values are the mean (± SEM) of triplicates.