The invention provides for a novel cholesterol loaded insect cell membrane preparation having an increased cholesterol level as compared to physiological cholesterol levels of insect cell membranes or to control insect cell membrane preparations without cholesterol loading, wherein said cholesterol loaded membrane preparation comprises an ABC transporter protein having an increased substrate transport activity due to increased cholesterol level of the membrane. The invention also relates to reagent kits comprising the preparations of the invention. The invention also relates to methods for manufacturing said preparations and methods for measuring any type of activity of the ABC transporters present in the cholesterol loaded membranes as well as studying or testing compounds and interaction of compounds and ABC transporters, in this assay systems. The invention also provides for a test system useful for testing whether ABC transporter proteins can be activated by cholesterol in an insect cell membrane.