We present hybrid quantum chemical-molecular dynamics computations on 15 partially hydrated proteins that support the view that the fluorescence wavelength maximum of a tryptophan (Trp) in a protein is determined almost entirely by the local electric field projection along the long axis of the indole ring, that is to say, an internal Stark effect. The response is 1.3 x 10(6) V cm(-1)/nm, with a shift to longer wavelength when the field forces electrons from the pyrrole to the benzene ring. Of 20 Trps simulated, representing a range of exposure to solvent, 15 are predicted within 10 nm of the experimental value. The findings should greatly aid the important task of finding effective methods for computing electrostatic fields and potentials within proteins.