1. The activity of acetylcholinesterase in the human erythrocyte membrane was measured with and without halothane. 2. To evaluate the roles of the supporting membrane lipids, the enzyme protein was solubilized from the membrane with a surfactant, Triton X-100. 3. It is confirmed that membrane lipids alter the activation energy of the enzyme bound to the membrane, and strengthen the effects of halothane and Triton X-100 on the enzyme activity by providing a high concentration field of them around the enzyme.