Spinach chloroplasts and subchloroplasts were treated with potato lipolytic acyl-hydrolase. The lipids were hydrolyzed through two steps. In the first step a major part of lipids (Type I) was rapidly hydrolyzed without affecting the photoactivity. In the second step the remaining lipids(Type II) were decomposed with a decrease of the photoactivity. The main components of Type II lipids in the photosystem I particles were monogalactosyldiglyceride, and those of the photosystem II particles were phosphatidylglycerol. Molecular species were clearly different between Type I and Type II of monogalactosyldiglyceride, but not in those of other lipid classes. It seems likely that Type II lipids are the bound lipids relating to the photoactivity and Type I lipids are the fluid bilayer lipids which have no concern with the activity.