It was recently shown that co-expression of adenovirus type 3 (Ad3) penton base and fibre in the baculovirus system produces dodecahedral particles, as does the expression of the penton base alone. The structure of both of these dodecahedral particles, with and without fibre, has been determined by cryoelectron microscopy and 3-dimensional reconstruction techniques to a resolution of 25 and 20 A, respectively. The general form of the penton base resembles that of the base protein in the recent reconstruction of adenovirus type 2. There is a remarkable difference in the penton base structure with and without the fibre. The five small protuberances on the outer surface of each base move away from the 5-fold axis by approximately 15 A when the fibre is present. These protuberances are of relatively low density and most probably represent a flexible loop possibly containing the RGD site involved in integrin binding. The fibre is apparently bound to the outer surface of the penton base, rather than inserted into it. The fibre is flexible and the shaft contains two distinct globular regions 26 A in diameter. The volume of the inner cavity of the dodecahedron is 350 +/- 100 nm3. This small volume precludes the use of the inner cavity to house genetic information for gene therapy; however, the possibility remains of linking the gene to the dodecahedron surface in the hope that it will be internalized with the dodecahedron.