The 26S proteasome is a multi-subunit protein complex which degrades proteins targeted for breakdown by ubiquitin conjugation. It plays a fundamental role by controlling the levels of highly important proteins such as regulators of the cell cycle and apoptosis. It is formed of a proteolytically active 20S core with two 19S regulatory particles (19S-RP) at each end. Despite its crucial importance, the molecular processes for the 19S-RP ubiquitin recognition, unfolding of the target protein and its translocation into the 20S core for degradation are still not fully understood. However this knowledge is required for the full explotation of the 26S proteasome as a therapeutic target. There is structural information already available on the 26S proteasome. The structure of the mammalian 20S core has been determined by x-ray crystallography (1) while the structure of the complete 26S proteasome has been studied by electron microscopy and image analysis (2). However, although the structural organisation of the 20S core is well established, it is still not clear how the different protein subunits within the 19S RP are organised, how they recognise and bind the ubiquitinated substrates, unfold the target protein or how they interact with the 20S core. We have obtained 3D structures of double (two 19S-RP capping the 20S core) and single capped (only one 19S-RP at one end of the 20S core) proteasome complexes, from human erythrocytes, by electron microscopy and single particle image analysis. The crystallographic structure of the 20S core can be docked into the resulting 3D map of the double-capped 26S proteasome by aligning the symmetry axis common to these structures. The correctness of this docking is further reinforced by inspection of the densities corresponding to each 20S core subunit. These subunits are characterised by detailed differences in their 3D structure, which in the 3D map of the double-capped 26S proteasome can be seen to match their corresponding individual shapes. The clear identification of subunits within the 26S proteasome 3D maps also allows the interactions between the 20S and the 19S complexes to be inferred. The comparison of the 3D maps obtained for the single and double capped forms of the proteasome reveals an opening of the entry channel into the 20S core associated with binding of the 19S-RP, which appears to be achieved by a small displacement to higher radius of each α-subunit resulting in a considerably better agreement between the densities of the 3D map in this region and the crystallographic coordinates of the 20S core. Furthermore, the current 3D maps of the 26S proteasome allow us for the first time to propose likely locations and spatial distributions of a number of the subunits of the 19S-RP.