In native gradient PAGE vitellogenin of Nauphoeta cinerea showed three closely spaced bands with molecular weights ( M r ) of 300,000, 314,000 and 329,000 and a broad band with a M r of approx. 600,000, and vitellin three closely spaced bands with M r s of 274,000, 293,000 and 308,000. Using the same technique vitellin of Leucophaea maderae appeared as several bands with M r s of 270,000–350,000, Hyalophora cecropia vitellin as a single sharp band with a M r of 520,000 and Manduca sexta vitellin as three bands with M r s of 510,000, 520,000 and 545,000. Investigation of the role of the pH and the ionic strength in the analytical ultracentrifugation revealed a M r of 244,000 for vitellin (sedimentation equilibrium method) and a s 20,w - value of 9.4 S when analyzed in PAGE buffer (pH 8.25), whereas a M r of 491,000 and a s 20,w of 15 S were found in 0.02 M sodium phosphate, 0.4 M NaCl, pH 7.5 At pH 3.6 aggregation of vitellin was observed. In sucrose density centrifugation addition of Triton X-100 altered the sedimentation coefficient from an apparent value of 15 to 8.4 S. These data suggest that the native vitellin is composed of two monomers with a M r of 244,000 which assemble to a dimer by hydrophobic interactions. Determination of the Stokes' radius by gel permeation chromatography revealed a value of approx. 7.5 nm for both the monomeric and dimeric form of vitellin and the fractional ratio was calculated to be 1.67 for the monomer and 1.45 for the dimer indicating an elongated shape. Immunodiffusion tests showed the presence of the same two components in vitellogenin and vitellin, one component being much more abundant than the other; data from gel permeation chromatography suggest that the minor component is a smaller molecule with a Stokes' radius of 4.8 nm. Comparative analyses of the carbohydrate, lipid and amino acid composition of vitellogenin and vitellin revealed no major differences although minor differences with respect to the phospholipid composition were observed.