Of the six chlorophyll-protein complexes of spinach and barley resolved by mild gel electrophoresis, two were chlorophyll a-protein complexes of PS I, namely CP1a and CP1, which accounted for up to 30% of the total chlorophyll. Both of these complexes had one P-700 per 120 chlorophyll a molecules. Since spinach and barley thylakoids have some 400 chlorophyll molecules per P-700, these complexes may not have lost any of the chlorophyll associated with them in vivo. This may account for CP1a and CP1 having the characteristic low-temperature fluorescence normally associated with PS I in vivo, which is not found in complexes with low chlorophyll/P-700 ratios. Two-dimensional electrophoresis showed that all of the chlorophyll a and P-700 of CP1 was bound to 70 kilodalton polypeptides. The PS I reaction centre complex of lowest mobility, CP1a, contained CP1 and four additional low molecular weight polypeptides. The three light-harvesting complexes resolved had major 25 and 23 kilodalton polypeptides. The presumed reaction centre complex of PS II contained major 50 and 47 kilodalton polypeptides.