Buoyant density sedimentations in CsCl of adenovirus type 5 and virus-antibody complexes has been described. It has been demonstrated that virus which survives a certain dose of antibody is found at densities lower than that of untreated particles suggesting a coating of the virus with antibody. It was further shown that the fraction of virus that was reactivated by treatment at pH 2.5 is still found at the low density, suggesting that pH 2.5, although capable to reactivate virus activity, is not sufficient to dissociate virus and antibody.