LoVo adenocarcinoma cells are fairly sensitive to cytostatic drugs, e.g. doxorubicin, but can develop drug resistance by expression of a P-glycoprotein-mediated MDR1 phenotype. LoVo cells respond with apoptosis to nanomolar concentrations of okadaic acid and micromolar concentrations of cantharidic acid. Interestingly, LoVoDx cells which had become about 10-fold less sensitive to doxorubicin by incubation in increasing concentrations of this cytostatic drug were also less sensitive to the toxicity of okadaic acid. Resistance to both agents was lost or significantly reduced by incubation in drug-free medium for about 4 months. On the other hand, LoVoDx cells did not lose responsiveness to the structurally different phosphatase inhibitor cantharidic acid but were about twofold more sensitive to the cytotoxic effect of this agent. Thus, MDR expression protects LoVo cells from the toxicity of phosphatase inhibitors that presumably are substrates of the P-glycoprotein, e.g. okadaic acid and its derivatives but not cantharidic acid, despite the fact that both agents are potent inducers of apoptotic cell death via ser/thr phosphatase inhibition.