Kreuz, Leo E. (The Johns Hopkins University School of Medicine, Baltimore, Md.), and Allan H. Levy. Physical properties of chick interferon. J. Bacteriol. 89:462–469. 1965.—The sedimentation coefficient, diffusion coefficient, and molecular weight of chick-embryo interferon were determined by zone centrifugation and equilibrium sedimentation in cesium chloride density gradients, and by chromatography on Sephadex G-100 columns. Purified interferon is not available in quantities sufficient to permit direct analysis by chemical or physical means; its relative concentrations were determined, therefore, by bioassay. I125-human serum albumin was used as an internal reference in all experiments. The sedimentation coefficient of chick-embryo interferon is 2.2 to 2.3S; the diffusion coefficient is 9.5 × 10−7 cm2sec−1. A molecular weight of 26,000 was calculated from the sedimentation and diffusion coefficients, and a range of 25,000 to 34,000 daltons was obtained from equilibrium-sedimentation analyses.