The 33 ku protein,prepared from NaCl-treated PSⅡ particles,has shown an single band by SDS-PAGE.After being dialyzed against the low-osmotic medium at 4℃,it has been found that the 33 ku protein degraded into several small fragments.This result suggests that the preparations of 33 ku protein probably contain some latent proteinases.It has also been found,by the 2-D electrophoresis and IEF,that the preparations of 33 ku protein not dialyzed against the low-osmotic medium contain several polypeptides with nearly the same molecular weight but not the same isoelectric point as the 33 ku protein.