Summary Factors affecting the determination of buoyant density in caesium salts of several animal picornaviruses have been studied. The values obtained for the acid-labile foot-and-mouth disease virus increased with the duration of the centrifugation and were higher at elevated pH values. In contrast, the acid-stable enteroviruses had the same values irrespective of the time of centrifugation or the pH of the caesium solution. Viruses of intermediate stability, such as vesicular exanthema virus and the rhinoviruses, were affected in the same way as foot-and-mouth disease virus but to a smaller extent. The implications of these observations in respect of the structure of the picornavirus group are discussed.