The membrane fatty acid composition of cancer cells can be modified either in culture or during growth in animals without disrupting basic membrane or cellular integrity. Only fatty acids are affected; no changes occur in membrane cholesterol, phospholipid, or protein content. There are changes in membrane physical properties and certain cellular functions, including carrier-mediated transport, receptor binding, ion channels, and eicosanoid production. Fatty acid modification also can enhance the sensitivity of the cells to hyperthermia and Adriamycin. This technique provides a new approach to understanding the membrane properties of neoplastic cells. Membrane fatty acid modification also may be of potential value as a therapeutic approach designed to augment the cytotoxicity of other antineoplastic therapies.