The dissolved macromolecules are represented by fractal aggregates of the same mass and hydrodynamic properties, composed of solid particles. The model presented gives the relation between sedimentation coefficient of an individual coil and that for the semidilute solution if the coil and blobs can be modeled by the same fractal aggregate. Using this model the sedimentation coefficient of an individual macromolecule is recalculated into the sedimentation coefficient of semidilute solution. The sedimentation coefficients are then compared by plotting them against the mass of object under consideration (macromolecule or blobs). The points for individual coils are situated lower. This incompatibility is analyzed in terms of the coil size reduction with concentration in the dilute region. The reduction of coil size corresponds to the increase of the size of non-porous particles forming the equivalent aggregate. In a good solvent such contraction can be also possible by reduction of the excluded volume exponent. (c) 2005 Elsevier Ltd. All rights reserved.