We have investigated the hypothesis that the antiproliferative effect of l-O-octadecyl-2-O-methylglycero-3-phosphocholine (ET-18-OCH3) is mediated through the inhibition of cellular acylation processes that control the unsaturated fatty acid complement of phospholipids. The effect of K'l -1K-<K11.i on the incorporation of radiolabeled oleic, linoleic, and arachidonic acids into MCF7 and T84 phospholipids was investi gated. Incubation of MCF7 cells with fatty acids and 2.75 fig/ml ET-18<K 'H.i, which inhibited the proliferation of the cells after 8 h, resulted in decreased incorporation of fatty acids into a number of phospholipids, notably phosphatidylcholine; however, increased incorporation of fatty acids into other phospholipids was also observed. After 12 h incubation with the alkyl-lysophospholipid, differences in the distribution of newly incorporated fatty acids into the phospholipid classes were observed without any effect on the total amount of fatty acid incorporated. Incu bation of MCF7 cells with 5 Â¿tg/mlK I-IK-OC II,, which caused a cessation in proliferation, had a similar effect on the incorporation of the fatty acids into the phospholipids, but the redistribution of newly incor porated fatty acids in the phospholipids was accompanied by a decrease in the amount of associated radiolabeled fatty acid. Incubation of IS4 cells with the labeled fatty acids and 3.5 jjg/ml KI-IS-Ã•KII,. which significantly decreased proliferation after 8 h, resulted in decreased incorporation of oleic acid into phosphatidylcholine and increased incor poration of oleic, linoleic, and arachidonic acids into phosphatidylethanolamine, prior to the decrease in proliferation. After 12 h incubation with alkyl-lysophospholipid, significant increases in the total amount of labeled oleic and arachidonic acids incorporated in the phospholipid fraction were observed. These results clearly indicate that the antiprolif erative effect of ET-18-OCHj in MCF7 and T84 cells is not dependent on inhibition of acylation processes and the above hypothesis may not be applicable to all alkyl-Iysophospholipid-sensitive cells.