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Light-harvesting Chl a/b protein complexes were isolated from the higher plant Sinapis alba, the green alga Chlorella fusca, and the prasinophycean alga Mantoniella squamata by mild gel electrophoresis. The energy transfer from chlorophyll b and the accessory xanthophyll was measured by means of fluoresence spectroscopy at 77 K. The pigment composition of the isolated antenna complexes was determined by high performance liquid chromatography in order to calculate the number of light absorbing molecules per chlorophyll a in the different light-harvesting complexes. These results were complemented by the quantitation of the pigments in total thylakoids as well as in the different electrophoretic fractions. On the basis of these data the in vivo ratios of xanthophylls per chlorophyll a could be estimated. The results show that the light-harvesting complexes from Chlorella and from Sinapis exhibit identical ratios of total xanthophylls per chlorophyll a. By contrast, in the prasinophycean alga Mantoniella, the light-harvesting complex markedly differs from the other chlorophyll b containing proteins. It contains, in addition to neoxanthin and violaxanthin, high amounts of prasinoxanthin and its epoxide, which contribute significantly to light absorption. The concentration of chlorophyll b in the complex is very much higher in the antenna of Mantoniella than in those of Chlorella and Sinapis. Furthermore, it must be emphasized that in addition to chlorophyll b, a third chlorophyll species acts in the energy transfer to chlorophyll a. This chlorophyll c-like pigment is found to be present in a concentration which improves very efficiently the absorption in blue light. In light of these results it can be concluded that the absorption cross section in Mantoniella is higher not only because of an enhanced number of light-harvesting particles in the membrane, but also because of a higher ratio of accessory pigments to chlorophyll a.