During the years, 1950-1960, there was a great deal of research that demonstrated the importance of lipids to insects and particularly their requirements for exogenous sources of sterols and the fatty acids, linoleic and linolenic. Phospholipid vesicles are prepared which enclose an aqueous solution containing the test materials. In insect cell cultures the sterols are required only as components of the cellular membranes since none of the established cell lines are known to synthesize hormones. The method of preparing the stock solutions of cholesterol was of critical importance for the successful long term culture of the cells. The need to supplement the existing cell culture media with sera, either invertebrate or vertebrate, has obscured any lipid requirement because the materials are naturally high in lipids. Continuously cultured insect cells are potentially very useful in the study of the metabolism of lipids.