Hen egg-white avidin was crystallized at pH 6 from solutions containing 8-9% (w/v) poly(ethyleneglycol) 6000 as precipitant. X-ray investigation showed that the crystals are orthorhombic with cell constants a = 7.47 nm, b = 8.00 nm, c = 4.31 nm and belong to space group P21212. They contain half of a tetrameric molecule in the asymmetric unit.