Plasma membranes purified 32- to 45-fold were isolated from leukaemic T-lymphoblasts, both sensitive and resistant to the Vinca alkaloid vinblastine. On development of drug resistance there was a very significant elevation of ether lipid content. 1-0-alkyl phospholipid increased by 200% with a smaller 30% increase in 1-0-alkenyl phospholipid. Cholesterol and phospholipid levels were also found to increase by 50% and 30% respectively, while the lipid to protein ratio increased by 60%. More modest changes were observed in the fatty acid composition of the membranes, with an alteration in the double bond index from 35.3 to 41.2. These lipid changes may have important implications in the changes to membrane permeability that develop with drug resistance.