Densonucleosis viruses (types 1 and 2) contain four major structural polypeptides with a total molecular weight in excess of the coding capacity of the DNA. Peptide maps obtained by limited proteolysis of isolated 125I-labeled proteins of both virus types indicate a common origin of the virus proteins and homology between the different viruses. The structure of densonucleosis virus type 2 and its homologous top component (naturally occurring empty particles) was compared by proteolysis using several proteases and the bifunctional cross-linking reagents dimethylsuberimidate (DMS) and dimethylmalonimidate. Similar susceptibilities of both components with proteases were obtained. The top components alone were accessible to the action of the cross-linking reagent DMS. The lowest molecular weight major structural polypeptide was most resistant to the action of the proteases and DMS.