It is argued that a knowledge of the extent of individual cell membrane systems is of crucial importance to the understanding of results obtained from lipid analysis of tissues and isolated membrane systems. In this preliminary study stereological methods have been applied to electron micrographs of cucumber leaves at different stages of expansion, to assess the area of each membrane system present. The equivalent phospholipid-protein membrane area for the largest leaf was found to be 500 × 109 μm2 compared with a value of 961 × 109 μm2 calculated from published data on the phospholipid content of a similar leaf. Possible applications of quantitative electron microscopy to certain lipid studies are reviewed and it is concluded that this approach should be more widely adopted.