(1) Chloroplast lamellae, like many other membranes, are composed of 90-A subunits. (2) The way in which the 90-A units associate with one another determines the quantasome structures of the membrane. There appear to be four 90-A subunits per quantasome in the most highly organized arrays. In at least some cases, the way in which the subunits bind one to another is under environmental control. (3) Extraction experiments are consistent with a model in which the membrane lipid surrounds the protein framework of the membrane.