The electrophoretic migratio rates of several proteins of photosystem 2 particles from spinach were much higher in gels containing 1m M Ca2+ than in gels containing 1m M ethylenglyocl-bis (β-aminoethyl ether) N,N,N1,N1,-tetraactic acid (EGTA). Incubation of gels with terbium (Tb3+) and the corresponding Tb3+-fluorescence were used to identify the Ca2+-binding proteins on the basis of selective occupation of Ca2+-binding sites with Tb3+. The 47, 43 and particularly 33 kDa polypeptides were most probably involed in Ca2+-binding.