Oxidized and cross‐linked proteins tend to accumulate in aging cells. Declining activity of proteolytic enzymes, particularly the proteasome, has been proposed as a possible explanation for this phenomenon, and direct inhibition of the proteasome by oxidized and cross‐linked proteins has been demonstrated in vitro. We have further examined this hypothesis during both proliferative senescence (this paper) and postmitotic senescence (see the accompanying paper, ref 1) of human BJ fibroblasts. During proliferative senescence, we found a marked decline in all proteasome activities (trypsin‐like activity, chymotrypsin‐like activity, and peptidyl‐glutamylhydrolyzing activity) and in lysosomal cathepsin activity. Despite the loss of proteasome activity, there was no concomitant change in cellular levels of actual proteasome protein (immunoassays) or in the steady‐state levels of mRNAs for essential proteasome subunits. The decline in proteasome activities and lysosomal cathepsin activities was accompanied by dramatic increases in the accumulation of oxidized and cross‐linked proteins. Furthermore, as proliferation stage increased, cells exhibited a decreasing ability to degrade the oxidatively damaged proteins generated by an acute, experimentally applied oxidative stress. Thus, oxidized and cross linked proteins accumulated rapidly in cells of higher proliferation stages. Our data are consistent with the hypothesis that proteasome is progressively inhibited by small accumulations of oxidized and cross‐linked proteins during proliferative senescence until late proliferation stages, when so much proteasome activity has been lost that oxidized proteins accumulate at ever‐increasing rates. Lysosomes attempt to deal with the accumulating oxidized and cross‐linked proteins, but declining lysosomal cathepsin activity apparently limits their effectiveness. This hypothesis, which may explain the progressive intracellular accumulation of oxidized and cross linked proteins in aging, is further explored during postmitotic senescence in the accompanying paper (1).—Sitte, N., Merker, K., von Zglinicki, T., Grune, T., Davies, K. J. A. Protein oxidation and degradation during cellular senescence of human BJ fibroblasts: part I—effects of proliferative senescence. FASEB J. 14, 2495–2502 (2000)