Photosystem (PS) II membranes, obtained by the method of Berthold et al. (Berthold, D. A., Babcock, G. T., and Yocum, C. F. (1981) FEBS Lett. 134, 231-234), have been fractionated by a sucrose gradient ultracentrifugation method which allows the quantitative separation of the three major chlorophyll binding complexes in these membranes: the chlorophyll (chl) a binding PSII reaction center core, the major light-harvesting complex II, and the minor chl a/b proteins called CP26, CP29, and CP24. Each fraction has been analyzed for its subunit stoichiometry by quantitative sodium dodecyl sulfate-polyacrylamide gel electrophoresis methods. The results show that 12 mol of light-harvesting complex II and 1.5 mol of each of the minor chl a/b proteins are present per mol of the PSII reaction center complex in PSII membranes. These data suggest a dimeric organization of PSII, in agreement with a recent crystallographic study (Bassi, R., Ghiretti Magaldi, A., Tognon, G., Giacometti, G. M., and Miller, K. (1989) Eur. J. Cell Biol. 50, 84-93) and imply that such a dimeric complex is served by antenna chl a/b proteins whose minimal aggregation state includes three polypeptides. This was confirmed by covalent cross-linking of purified antenna complexes.