Purified polyoma virus can take up an amount of calf thymus histone equivalent to 10-50% of its normal histone content under conditions allowing the binding of considerably lesser amounts of several other proteins. Some of the bound histone could not be released by procedures routinely used for virus purification. Some of the histone present in purified polyoma virus could be selectively released without major breakdown of virus particles. Possible models for virus structure are discussed in the light of the present and other recent data.