Chicken egg-white lysozyme forms a yellow complex with N-methylnicotinamide chloride. Titration studies utilizing the appearance of the yellow color as a measure of complex formation indicate that the weak complex (association constant k = 3.2 liter mole(-1)) involves a single class of binding sites on the lysozyme molecule. By analogy with similar titration studies on model compounds containing the indole moiety, the site for N-methylnicotinamide binding is probably the indole ring of a single, solvent-available tryptophan residue. The yellow color itself apparently arises from a charge transfer transition, with the indole ring system serving as the donor and N-methylnicotinamide as the acceptor. Complete resolution of the charge transfer spectrum of the lysozyme-N-methylnicotinamide complex was not achieved due to the very high absorbance of the protein near the short-wavelength absorption edge of the band. However, it is possible to consider the spectrum as the sum of two Gaussian bands whose positions and relative intensities agree remarkably well with the positions and relative intensities obtained by Gaussian fitting of the charge transfer spectra of several model complexes between substituted indoles and N-methylnicotinamide. The geometry for such complex formation requires that the ring faces of both donor and acceptor be more or less completely available for complexation. The possible use of N-methylnicotinamide as a molecular probe for tryptophan residues having at least one indole ring face freely available to the solvent is discussed.