A method whereby lactoperoxidase-catalyzed 125I-iodination of plasma membrane lipids can be achieved is described. The reaction results in a uniform and stable labeling of neutral lipids, phospholipids, lysophosphatides, free fatty acids, and triacylglycerides. By the use of this method, the action of antibody plus complement (C) on the specific release of lipid from the plasma membrane of line-10 tumor cells was studied. Within 15 min after the addition of C to antibody-sensitized cells, the enhanced release of specific lipid classes from the cell surface was observed; these lipids included sphingomyelin, phosphatidylserine, and phosphatidylcholine. The release of phosphatidylethanolamine and, in some instances, triglycerides, was reduced after antibody-C attack. Neither the specificity of the antibody used to sensitize the cells nor the ability of the antibody plus C to be cytotoxic to the cells appeared to affect the identity or amounts of lipids released from the cells.