PSII particles from Chlamydomonas reinhardtii were purified according to the protocol of Diner and Wollman (Diner, B.A., and Wollman, F.-A. (1980) Eur. J. Biochem. 110, 521-526) followed by ion-exchange chromatography. They contained the psbA, psbB, psbC, and psbD gene products in a 1/1/1/1 stoichiometry, cytochrome b559, and several small polypeptides, and exhibited electron transfer from donor Z to acceptor QA (40-50 chlorophylls/reducible QA). Upon ultracentrifugation and molecular sieving in the presence of either lauryl maltoside or octaethylene glycol dodecyl ether (C12E8), they behaved as monomers of 440-510 kDa, including the detergent. C12E8 preparations also contained a small proportion of a partially interconvertible dimeric form. Four small subunits were identified by N-terminal sequencing, namely a 6.1-kDa nuclear-encoded subunit and three chloroplast-encoded subunits homologous to psbE, psbK, and psbM gene products. Cytochrome b559 subunit alpha (psbE) of C. reinhardtii, but not subunit beta (psbF), was recognized by an antiserum raised against higher plant cytochrome b559. The products of the psbF, psbI, and psbN genes remained undetected, presumably because of blocked N termini. At least four polypeptides presented both phosphorylated and unphosphorylated forms (psbC, psbD, and psbH gene products, as well as an unidentified 5-kDa subunit).