Oxidized cytoplasmic and nuclear proteins are normally degraded by the proteasome, but accumulate with age and disease. We demonstrate the importance of various forms of the proteasome during transient (reversible) adaptation (hormesis), to oxidative stress in murine embryonic fibroblasts. Adaptation was achieved by ‘pre-treatment’ with very low concentrations of H₂O₂, and tested by measuring inducible resistance to a subsequent much higher ‘challenge’ dose of H₂O₂. Following an initial direct physical activation of pre-existing proteasomes, the 20S proteasome, immunoproteasome and PA28αβ regulator all exhibited substantially increased de novo synthesis during adaptation over 24 h. Cellular capacity to degrade oxidatively damaged proteins increased with 20S proteasome, immunoproteasome and PA28αβ synthesis, and was mostly blocked by the 20S proteasome, immunoproteasome and PA28 siRNA (short interfering RNA) knockdown treatments. Additionally, PA28αβ-knockout mutants achieved only half of the H₂O₂-induced adaptive increase in proteolytic capacity of wild-type controls. Direct comparison of purified 20S proteasome and immunoproteasome demonstrated that the immunoproteasome can selectively degrade oxidized proteins. Cell proliferation and DNA replication both decreased, and oxidized proteins accumulated, during high H₂O₂ challenge, but prior H₂O₂ adaptation was protective. Importantly, siRNA knockdown of the 20S proteasome, immunoproteasome or PA28αβ regulator blocked 50–100% of these adaptive increases in cell division and DNA replication, and immunoproteasome knockdown largely abolished protection against protein oxidation.