Desmin protofilaments and the proteolytically derived α-helical rod domain have been characterized by highresolution gel permeation chromatography (GPC) using columns calibrated for the determination of viscosity radii. Additional characterization by chemical cross-linking and the determination of sedimentation values allowed the calculation of the molecular dimensions of the molecular species isolated. In dilute buffers GPC separated desmin rod preparations into two complexes: a dimer species (single coiled coil) with a length of 50 ± 5 nm and a tetramer species (two coiled coils) with a length of 65 ± 5 nm. Thus the two coiled coils in the tetramer are staggered by approximately 15 nm. The hydrodynamically derived lengths of the rod dimer and tetramer are supported by electron microscopy after metal shadowing. The hydrodynamic properties of desmin protofilaments follow that of the rod tetramer. The data on the hydrodynamic analysis of the rod tetramer of desmin in solution are in full agreement with the structural information recently deduced from paracrystals of the rod of glial fibrillary acidic protein [Stewart, M., Quinlan, R. A. & Moir, R. D. (1989) J. Cell Biol. 109, 225–234]. Our results explain the inhomogeneity of molecules encountered in previous electron microscopical analyses.