Platelets contain a lipoxygenase which converts arachidonic acid to 12-L-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) which has been shown to be chemotactic for human neutrophils and eosinophils. [14C]-12-HETE was biosynthesized, purified and incubated at a concentration of 1 μM with human neutrophils. Lipids were extracted from the neutrophils and the media, and the radiolabeled products identified. 26 percent of the radiolabel was found in the cells after 30 min incubation, essentially all of it esterified into phospholipid and triglyceride. The radiolabeled phospholipids and triglycerides were transesterified and the liberated fatty acid was identified as [14C]-12-HETE. This is the first demonstration of direct alteration of membrane components by a chemotactic agent and may be an example of a more generalized mechanism for altering membrane characteristics.