Highly active photosystem-II particles were rapidly isolated using detergents and obtained in good yield from a mutant of the green alga Chlamydomonas reinhardtii. The particles are completely devoid of reaction centers of photosystem I, and of the secondary electron acceptor to photosystem II. They show: (a) a specific activity (delta A of C550/unit chlorophyll) 4--7 times that of the starting material and of spinach chloroplasts: (b) an antenna size of 40 to 50 chlorophyll molecules containing little light-harvesting chlorophyll a/b complex (chlorophyll a/chlorophyll b = 4--6.4); (c) a ratio of variable to dark-adapted fluorescence yield of up to 3. Further treatment of these particles by ion-exchange chromatography largely removes five proteins and further decreases the antenna size with little loss in primary photoactivity.