A PSII reaction center complex consisting of three polypeptides. D1. D2 and Cytb559. was first purilied from broad bean leaves. The complex fairly activy DCIP photoreduc-tion. In the presence of DPC, and showed signal 1Is either in the dark or under illumination The complex also contained manganesc atoms its Mn2+EPR Intcnsity decreased by about 40% under continuous i11umination and recovered to the original level when the complex was transferred to the dark. the above results indicated that the complex reported here contains all of the PSII electron transport chaincomponents from the secondary donor Z to the stable primary electron acceptor QA. and it is possible that the complex contains manganese binding sites. the alternation in dark and illumination can induce reversible valence changes of manganese atoms in the purified complex.