Rate constants have been determined for the electron-transfer reactions between reduced free flavins and flavodoxin semiquinone and several blue copper proteins. Correlations between these values and redox potentials demonstrate that spinach plastocyanin, Pseudomonas aeruginosa azurin, Alcaligenes sp. azurin, and Alcaligenes sp. nitrite reductase have the same intrinsic reactivities toward free flavins, whereas stellacyanin is more reactive (3.3 times) and laccase considerably less reactive (approximately 12 times). Electrostatic interactions between the negatively charged flavin mononucleotide (FMN) and the copper proteins show that the interaction site charges for laccase and nitrite reductase are opposite in sign to the net protein charge and that the signs and magnitudes of the charges are consistent with the known three-dimensional structures for plastocyanin and the azurins and with amino acid sequence homologies for stellacyanin. The results demonstrate that the apparent interaction site charge with flavodoxin is larger than that with FMN for plastocyanin, nitrite reductase, and stellacyanin but smaller for Pseudomonas azurin. This is interpreted in terms of a larger interaction domain for the flavodoxin reaction, which allows charged groups more distant from the actual electron-transfer site to become involved. The intrinsic reactivities of plastocyanin and azurin toward flavodoxin are the same, as was the case with FMN, but both stellacyanin and nitrite reductase are considerably less reactive than expected (approximately 2 orders of magnitude). This result suggests the involvement of steric factors with these latter two proteins which discriminate against large reactants such as flavodoxin.