ULTRACENTRIFUGAL studies have shown that purified diphtheria toxin is a single protein with a molecular weight of 64,500 and an S20, w of 4.2 (ref. 1). In this communication, I report experiments with a sample of toxin that contains the 4.2 S molecule, and an unusual species with a higher sedimentation constant. These two toxin molecules have been separated by disc electrophoresis on polyacrylamide gels, by ultracentrifugation and by chromatography on a thin layer of ‘Sephadex G-100’.