Photosystem I1 membrane fractions from dark-adapted mesophyll chloroplasts of maize were solubilized in different concentrations of dodecyl P-D-maltoside. Chlorophyll-binding proteins from photosystem I1 were isolated either by ultracentrifugation on a sucrose gradient, or by flat bed isoelectric focusing and identified by gel electrophoresis analysis for their polypeptide composition. Lipid and fatty acid compositions were determined in complexes prepared by both methods and also in purified light-harvesting complex 11, in minor chlorophyll alb binding complexes 29, 26, 24, in photosystem I1 antennae (chlorophyll-protein complexes 43, 47) and in the photosystem I1 reaction centers chlorophyll-protein complexes. Comparative analysis of the results suggests that a true heterogeneity exists in the lipid class distribution among the different chlorophyll-protein complexes in this region of the photosynthetic membrane. Photosystem I1 core fractions prepared either by ultra-centrifugation on a sucrose gradient or by isoelectric focusing were found significantly enriched in monogalactosyldiacylglycerol ; fractionation of the photosystem I1 core in its components showed that it was the chlorophyll-protein complexes 43 and 47 which were mainly responsible for this enrichment. One of them, the chlorophyll-protein complex 47, was found containing monogalactos yldiacylglycerol and having a very high level of saturated fatty acids. The minor chlorophyll alb binding linkers (chlorophyll-protein complexes 24, 26 and 29) retain a largely higher amount of lipids than all other complexes and especially of highly unsaturated galactolipids.Concerning the main light-harvesting antenna (LHCII), it is demonstrated that phosphatidylglycerol is strongly linked to the complex if it cannot be detached at high detergent concentration, while many galactolipids (which nevertheless represent the major lipid classes) are lost. This main light-harvesting complex has been fractionated into several families by isoelectric focusing showing a marked difference in lipid and polypeptide composition. A spectacular increase in the phosphatidylglycerol content was observed in the fraction migrating near the anode and enriched in a 26-kDa polypeptide; but this result is difficult to interpret in physiological terms as it was shown that phosphatidylglycerol alone, because of its negative charge, also migrates toward the anode in isoelectric focusing.In photosynthesis, the two major functions of light-harvesting and electron transport occur in a thylakoid which is the predominant membrane in the plant leaf cell representing up to 80% of the total cellular membranes (Forde and Steer, 1976). Thylakoids are organized into two major functional domains. The appressed or granal membranes and the nonappressed or stromal membranes contain different complements of electron transport and binding complexes participating in the photosynthetic process. The lipid composition of the thylakoid is unique since it always contains four lipids never found in other cellular m...