Single-cell partition analysis is described as a novel technique for examining ligand-macromolecule interactions. This procedure is a combination of the classical fluorescence titration technique and phase-partition techniques and allows three separate methods for calculating and comparing both free and bound drug concentrations. The value of this technique is demonstrated by the comparison of the binding properties of the potent antitumor antibiotic adriamycin and ethidium bromide to nucleic acids. Binding isotherms of both drugs were obtained at low r (concentration of bound drug per base pair) values, showing strikingly different results, thus allowing insight to be gained into the cooperative binding of these drugs to DNA. © 1983.