The mechanism of the photochemically induced nucleation of lysozyme was investigated. Radical cations and neutral radicals of tryptophan residue of lysozyme were observed by transient absorption measurements. The radical cation was found to be in equilibrium with the neutral radical, and the pK(a) of the radical cation in UV-vis wide range buffer solution was estimated to be ca. 3.2. The dimer formation was confirmed by SDS-PAGE for the solutions with pH value higher than this pKa value. The photochemically induced nucleation was also observed for these solutions. The number of crystals increased with increase of the dimer.