The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of two aromatic dyes (2,2'-dipyridyl, DP, and flavin mononucleotide, FMN) with bovine and human alpha-lactalbumins (BLA and HLA) in aqueous solution have been studied using the time-resolved CIDNP technique. With DP as the photosensitizer, polarization in BLA is observed for the protons of Trp 118, His68, Tyr18, and Tyr103. The latter is not manifested in the CIDNP spectra obtained with FMN. This threshold effect on CIDNP formation indicates that accessibility is a combined property of both the residue and the dye. The nuclear spin-lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics: T-1 = 60 mus for H2,4,7 of Trp118, T-1 = 53 mus for H3,5 of Tyr103, T-1 = 16 mus for H3,5 of Tyr18, and T-1 = 10 mus for the H2 and H4 of His68. The correlation times for the side chain motion as determined from the T, of the radicals are correlated with the accessibility of the side chains in the intact protein.