PA28 is a γ‐interferon‐induced complex that associates with the 20S proteasome and stimulates breakdown of small peptides. Recent immunoprecipitation studies indicate that, in vivo, PA28 also exists in larger complexes that also contain the 19S particle, which is required for ATP‐ubiquitin‐dependent degradation of proteins. However, because of its lability, the structure and properties of this larger complex remain unclear. Here, we demonstrate that, in vitro, PA28 can associate with'singly capped’26S (ie 19S–20S) proteasomes. Electron microscopy of the resulting structures revealed one PA28 ring at one end of the 20S particle and a 19S complex at the other. These hybrid complexes show enhanced hydrolysis of small peptides, but no significant increase in rates of protein breakdown. Nevertheless, during breakdown of proteins, the complexes containing PA28αβ or PA28α generated a pattern of peptides different from those generated by 26S proteasomes, without altering mean product length. Presumably, this change in peptides produced accounts for the capacity of PA28 to enhance antigen presentation.