The cytoplasmic concentration of ionised calcium, [Ca2+]i, is believed to be altered by agents that induce a malignant hyperthermia (MH) crisis in susceptible individuals. MH patients were identified by the halothane and halothane/caffeine contracture tests done in isolated muscle biopsy specimens. [Ca2+]i was measured in isolated peripheral blood mononuclear cells from MH patients and controls by means of the fluorescent calcium ion indicator quin2. In the absence of halothane there was no significant difference in [Ca2+]i in cells from normal and MH patients. Addition of halothane (4 microliter/ml) significantly increased [Ca2+]i in cells from MH patients but not in controls. The halothane-induced increase in [Ca2+]i required extracellular calcium ions. This is the first evidence of the mechanism of action of halothane in cells of MH patients; the differential effect of halothane on [Ca2+]i might constitute the basis for a non-invasive screening test for MH.