Dans, P. E. (National Institute of Allergy and Infectious Diseases, Bethesda, Md.), B. R. Forsyth, and R. M. Chanock. Density of infectious virus and complement-fixing antigens of two rhinovirus strains. J. Bacteriol. 91:1605-1611. 1966.-Two rhinovirus serotypes (echovirus 28 and HGP) and poliovirus type 1 were banded by isopycnic centrifugation in cesium chloride. The rhinovirus virions had a density of 1.41 g/ml, whereas that of poliovirus was 1.34. Since a number of other enteroviruses also have a density of 1.34 g/ml in cesium chloride, a basic difference in density may exist between the rhinovirus and enterovirus subgroups of the picornavirus family. Whether this difference reflects differences in ribonucleic acid content or binding of cesium ions remains to be determined. In tests with echovirus 28 two peaks of CF activity were detected: one in association with the virion (1.41 g/ml), and a larger peak of lower density (1.30 g/ml). With echovirus 28 antiserum, a heterotypically reactive complement-fixing (CF) antigen was detected in the HGP virus suspension at a density less than that of the virion (1.30 g/ml). This antigen corresponded in density to the less dense CF antigen of echovirus 28.