In order to identify protein complexes consisting of the proteasome and specific proteasome regulators, crude soluble lysates of red blood cells were fractionated by gel filtration chromatography and by velocity sedimentation centrifugation. The fractionated lysates were then tested for the relative distribution of proteasome activity, proteasome protein, and protein of a known proteasome activator, PA28. At least two proteasome complexes containing PA28 were identified. One of these complexes had an apparent molecular weight of approximately 1,750,000, and appeared to have much more proteasome activity than could be accounted for by its relative concentrations of proteasome and PA28 protein. We hypothesized that this complex contained another activator of the proteasome, and we sought to purify this activator from extracts of red blood cells. A proteasome activator with an apparent molecular weight of approximately 700,000 was identified, purified, and characterized. This activator, termed PA700, greatly stimulated the peptidase activities of the proteasome in an ATP-dependent fashion. PA700 was composed of about 16 polypeptides ranging in molecular weight from 20,000 to 100,000. The ATP-dependent activation of the proteasome by PA700 was closely linked to the formation of a high molecular weight complex that required no additional ATP for activated proteolysis. These results indicate that PA700 is a regulatory protein of the proteasome and is a component of at least one high molecular weight proteasome-containing complex occurring in cell extracts.