AbstratPeripheral blood mononuclear (PBM) cells from patients with halothane hepatitis are unusually susceptible to damage from phenytoin metabolites generated by anin vitrodrug metabolising system. In order to provide more information about the nature of this susceptibility factor, the effect of removing calcium ions (Ca2+) from the incubation medium of the test system was examined. Phenytoin metabolites were generated by incubating phenytoin with β‐naphthoflavone‐induced rat liver microsomes in the presence of 1,1,1‐trichloropropene oxide (TCPO), an epoxide hydrase inhibitor. When PBM cells from patients who had recovered from halothane hepatitis were incubated in this system and then maintained in Ca2+‐containing tissue culture medium (without α‐tocopherol) for 16 h, cell death, as measured by trypan blue exclusion, was greatly increased (53% and 78% at 0.06 mmol/1 and 0.12 mmol/1 phenytoin, respectively) compared with control incubations (TCPO omitted). Removal of Ca2+from the tissue culture medium effectively abolished reactive metabolite‐induced cell death. Resting cytosolic free Ca2+concentration in PBM cells was also measured using the quin‐2 fluorescence method and total Ca2+content was measured by atomic absorption spectrometry. Although variability appeared greater among patients, mean values for these parameters among 12 patients with halothane hepatitis did not differ from controls. It is concluded that enhanced permeability of PBM cells to extracellular Ca2+may be an important factor in the pathogenesis of drug metabolite‐induced cell death in patients susceptible to halothane hepatitis. Such permeability to Ca2+is not evident in resting cells and presumably results from an interaction between electrophilic metabolites and the pumps which regulate cell ca