Photosystem II complexes were prepared from chloroplasts of wild type tobacco Nicotiana tabacum var. John William’s Broadleaf and from two chlorophyll mutants derived from it, namely N. tabacum Su/su and N. tabacum Su/su var. Aurea. The hydrophobic peptides of these complexes were analyzed for bound lipid molecules by means of monospecific lipid antisera. A comparison of the peptide composition of the complexes of the three chloroplast types by means of polyacrylamide gel electrophoresis showed that the peptide composition was qualitatively identical. A major quantitative difference referred to a 66 kDa peptide which appeared to be much stronger in gels of photosystem II peptides originating from the yellowgreen and the yellow tobacco variety. Furthermore, we were able to show that different SDS polyacrylamide gel electrophoresis runs of the same PS II preparation yielded differences in the band strength of this peptide. Comparative densitometric measurements showed that an increase in this 66 kDa peptide was always correlated with a decrease in the D 1 and D 2 peptides. Obviously, the 66 kDa peptide is the heterodimer of D 1 and D 2 . Differences in the peptide composition of photosystem II preparations from the 3 tobacco species refer above all to peptides of the light-harvesting complex with molecular masses of 28 and 26 kDa. After the transfer of the peptides from the polyacrylamide gel to nitrocellulose membranes, they were incubated with monospecific antisera to monogalactolipid, digalactolipid or sulfolipid. These experiments showed that the 66 kDa peptide reacted with antibodies to digalactolipid and with those to sulfolipid. The 66 kDa peptide reacts in the Western blot procedure also with an antiserum to a 66 kDa peptide prepared and characterized earlier and which was shown to inhibit electron transport reactions in the region of the reaction center of photosystem II. The monospecific antiserum to monogalactolipid reacts with the D 1 and D 2 peptide as well as with the chlorophyll-binding polypeptides of the masses 42 and 48 kDa, and also with the 26 and 28 kDa peptides of the light-harvesting complex as well as with the extrinsic peptides exhibiting the molecular masses, 33 ,21 -23 and 18 kDa. Whereas lipase treatment apparently destroys the lipids as antigenic determinants of the peptides on the nitrocellulose membrane, periodate treatment or treatment of the photosystem II preparations with organic solvents do not prevent the reaction of the 66 kDa peptide with the sulfolipid antiserum. These experiments show as the 66 kDa peptide appears to be the heterodimer of D 1 and D 2 , that the galactolipids mono- and digalactosyldiglyceride as well as the sulfolipid are bound, much like prosthetic groups, onto the core peptides.