A water-insoluble organic 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical is solubilized. in aqueous solutions of aggregates of amphiphilic block copolymers with hydrophobic dendritic pendants. The number (100-200) of DPPH molecules solubilized in an aggregate, which is evaluated from UV-visible absorption spectra, is in agreement with that of the DPPH radicals determined from electron paramagnetic resonance (EPR) spectroscopy. The DPPH radicals are stably solubilized without decomposition in the polymer aggregates. The radicals exhibit a single-line EPR absorption, which is narrowed by the interspin interaction, and indicates the assembly formation of DPPH radicals in polymer aggregates. These results suggest the effective utilization of the DPPH radical as a spin-probe indicator in aqueous solutions.