Proteasomes and ubiquitin (Ub) are essential components of the energy-dependent, nonlysosomal proteolytic pathway. To clarify the physiological role of this proteasome/Ub-dependent pathway, we meaured the levels of expressions of proteasomes and Ub in human renal cancers by Northern blot and immunochemical analyses. The mRNAs for two of the multiple subunits of proteasomes, C2 and C9, were expressed at abnormally high levels in most neoplastic lesions of patients with various primary renal cell carcinomas and in all renal cancer cell lines examined. However, no significant difference was found by enzyme immunoassay in the proteasomal contents of cancerous and normal parts of the kidney. The levels of mRNAs for the subunits of proteasomes were high in rapidly proliferating renal cells and appeared to be correlated with the activities of these cells for proteasome synthesis, but the cellular contents of proteasomes in these cells were normal, suggesting rapid turnover of proteasomes in rapidly proliferating cancer cells. Consistent with the increased expressions of proteasomal mRNAs, the expressions of three Ub genes, mono-UbA80, mono-UbA52, and poly-UbC, were found to be greatly increased in these renal cancer cells. Immunohistochemical staining of normal kidney showed that the levels of both proteasomes and Ub were high in cells of renal tubules and collecting ducts, but low in the glomerulus. The levels of both proteins appeared to be considerably increased in the nuclei of granular and clear carcinoma cells of the kidney. Moreover, the profiles of cellular proteins conjugated with Ub in normal kidney tissues were different from those in cancerous parts of the kidney and in established renal cancer cells. These results suggest that the proteasome- and ubiquitin-mediated system is functionally involved in the cancerous state in human kidney.