We show through hen egg-white lysozyme nucleation experiments under light produced by a 300 W continuous Xe lamp that nucleation is enhanced. The irradiation of supersaturated protein solutions in NaCl at pH 4.5, β ) 9, from 10 to 60 s increased the number of lysozyme crystals in the droplet. The most effective irradiation time is from 10 to 30 s, and in this range, enzyme activity is not affected. The resulting crystal lattice parameters are identical to that normally obtained. We propose the following mechanism: intermediates produced by one photon absorption produce lysozyme radicals, which enhance nucleation. These results suggest a new method for controlling nucleation and growth that could be used in structural genomics and the pharmaceutical industry, for instance.