Colorless apoferritin (M congruent to 450 kDa) crystals were grown in the presence of red colored holoferritin dimers (M congruent to 900 kDa) within agarose gel by counterdiffusion with Cd2+ as precipitant. In agreement with our previous quantitative measurements showing preferential trapping of dimers (segregation coefficient K = 4), the apoferritin crystals became darkly colored while the gel solution around them was nearly colorless. This segregation coefficient and the absence of convective mixing resulted in a large colorless area surrounding the crystals which allowed us to demonstrate the existence of the impurity depletion zone by direct visual observation.