The ionic mechanisms and cytoprotective activities of 4-piperidinomethyl-2-isopropyl-5-methylphenol (THPI), an analogue of thymol, were investigated in HL-60 granulocytes and in human erythrocytes, respectively. THPI inhibited K+ outward current (IK) in a concentration-dependent manner in HL-60 leukocytes, with an IC50 value of 4 μM. Neither iberiotoxin (200 nM) nor paxilline (1 μM) suppressed the amplitude of IK, whereas clotrimazole (5 μM) significantly inhibited it. In the inside-out configuration of single channel recordings, application of THPI (5 μM) into the bath medium did not alter the single-channel conductance of intermediate-conductance Ca2+-activated K+ (IKCa) channels (i.e KCa3.1 channels), but it suppressed the channel activity significantly. THPI-induced inhibition of IKCa channels was reversed by a further application of 1-ethyl-2-benzimidazolinone (10μM). THPI-induced reduction in IKCa-channel activity in these cells was primarily due to a decrease in mean open time. These results provide direct evidence that THPI is capable of suppressing the activity of IKCa channels in HL-60 cells. The antioxidant action of THPI also revealed a beneficial cytoprotective effect against mitomycin C-mediated haemolytic effect in human erythrocytes. The results of this study suggest that blockade of IKCa channels and the membrane-protecting activity of THPI would combine to have beneficial effects in lessening the severity of haemolytic crisis and reducing anaemia in sickle cell disease.