A method is described of using photography to measure the concentrations of a small ligand (proflavine) above and below the boundary of a macromolecule (DNA, both native and denatured) sedimenting in the ultracentrifuge. The measurements are used to determine the extent of the binding of proflavine to DNA, and the results compared with those obtained by a spectrophotometric method. The results obtained by the two methods agree within 10%, thus validating the spectrophotometric technique under these conditions. The variation of the sedimentation coefficient with the extent of binding of proflavine was also studied. The results are discussed in relation to previously observed changes in the viscosity of the solutions.