Chromoplasts of unfolding petals of Tropaeolum majus contain large amounts of filaments (which, in sections, appear as tubules), and unevenshaped, isodiametric to elongated bodies (IBs). These structural elements are the major sites of the chromoplast pigments. They were freed from isolated chromoplasts and subjected to sucrose density gradient centrifugation. At a density of 1.080 g cm(-3) a distinct orange band contained almost exclusively fine filaments of 15-20 nm in diameter as shown after negative staining. Other filaments and most of the IBs were heterogeneous in size, shape, and density and were collected in two fractions of buoyant densities of 1.025 and 1.055 g cm(-3). The three fractions thus obtained comprise 15-33% protein, large amounts of carotenoids and their esters, glyco- and phospholipids, as well as minor amounts of tocopherols. A higher buoyant density of particles is correlated with a higher relative content of protein and glyco- and phospholipids and a lower relative content of carotenoids. The polypeptide pattern, as shown by SDS-polyacrylamide gel electrophoresis, is very similar in all three fractions. There is one main polypeptide, with a MW of about 30,000, accounting for up to 80% of the protein of each fraction.