The hydrodynamic radius of a membrane protein, bc(1)-complex, in aqueous solutions was investigated using a dynamic light scattering technique. The essential unit in the solutions was found to take the dimer form of bc(1)-complex molecules. The aggregates exceeding 100 nm appeared with an increase in the crystallizing reagent, PEG, and the precipitation of an amorphous phase was observed. This feature was characteristic of zinc-free solutions. When 2 mM zinc was in the solutions, large aggregates of proteins were not observed, and single crystals of bc(1)-complex directly nucleated when the PEG concentration reached a critical level. The mutual diffusion coefficient of the dimers in the zinc-free solutions decreased with an increase in the protein concentration when the concentration of PEG was constant, indicating an attractive force between molecules. When the solution contained zinc, the coefficient increased with an increase in the protein concentration, indicating a repulsive force between molecules.