The four first generations of dendrigraft poly-L-lysine have been studied in dimethylformamide (aprotic solvent) and in 0.2 M NaCl aqueous solutions by isothermal translation diffusion, 1H NMR and viscometry methods. The relationships between diffusion coefficient, intrinsic viscosity and molar mass have been determined for dendrigraft poly-L-lysines, and the scaling index values have been compared to classical trifunctional dendrimers. Dendrimers and dendrigraft poly-L-lysines exhibited similitudes in their hydrodynamic behaviors. Nevertheless, dendrigraft poly-L-lysines displayed a specific behavior in solution. In contrast to dendrimers, a significant change of hydrodynamic dimension of dendrigraft poly-L-lysines according to the nature of the solvent has been observed. In aprotic solvent, the dendrigraft poly-L-lysine dimensions are about two times lower than in aqueous media (i.e., the hydrodynamic volume is contracted by a factor 8 in dimethylformamide), revealing the softness of dendrigraft poly-L-lysine compared to classical trifunctional dendrimers.