The plasma membrane, which separates intracellular contents from extra cellular milieu, consists of a lipid bilayer comprising mainly phospholipids and cholesterol together with various functional proteins, which control the interaction of the cell with its environment. Cells are killed at elevated temperatures and previous work suggests that the plasma membrane may be a primary target in this hyperthermic cell death. The present study set out to test this hypothesis using a rat liver tumour cell hue (Hepatoma Tissue Culture cells). Several different experimental approaches were adopted. Supplementation of these cells with linoleic acid (18 : 2) for a 36 hour period increased the thermal sensitivity of cells at 43 C, though increased sensitivity was not evident at other times. Plasma membrane-enriched fractions were obtained from control cells and from cells supplemented with linoleic acid for a 36 hour period, then lipids were extracted and characterised. Whilst there was little difference in the cholesterol : phospholipid ratio, the phospholipid fatty acid composition of membranes from supplemented cells showed elevated levels of 18 : 2 and decreased levels of oleic acid (18 : 1) relative to control cell membranes. DPH fluorescence polarisation studies indicated that plasma membranes from supplemented cells were less 'ordered' than control membranes. Alkaline phosphodiesterase I, a plasma membrane-bound enzyme, appeared to be more thermolabile in supplemented cells suggesting that plasma membrane 'fluidity' may be an important factor in determining the thermal sensitivity of this membrane-bound enzyme. Hyperthermic cell death was potentiated by the presence of local anaesthetics, two of which, dibucaine and tetracaine, also produced less 'ordered' membranes. Morphological studies conducted on cells in the presence and absence of local anaesthetic at elevated temperatures indicated changes in cellular surface morphology on heating which were accelerated in the presence of the anaesthetic. The intermediate filament network of these cells did not appear to be a primary target of hyperthermic treatment. These studies suggest that the lipid composition and physical state of the plasma membrane are critical features involved in the expression of cell death, possibly through a modulation of membrane protein thermal sensitivity.