Twenty percutaneous renal transplant biopsies and 20 removed allografts were investigated ultrastructurally. Most of the detected alterations were of a degenerative or regenerative nature and not specific of rejection. The most interesting phenomenon was the tubulitis, namely, the migration of the interstitial inflammatory cells (IC) through the tubular basement membrane (BM) and the invasion of the tubular epithelium in this way. Tubular epithelial cells (TEC) in the vicinity of IC were often necrotic. The composition of cells invading the tubules corresponded to those infiltrating the interstitium. The distal tubule was more frequently infiltrated than the proximal tubule. The TEC were always in very close contact with the BM. The invading IC were in direct contact with the inner surface of the BM only while passing through it. IC that passed the BM were immediately separated from it by a thin epithelial layer. The tubular ultrastructural changes did not reveal substantial differences between the various rejection types, except for the pronounced thickening and lamellation of the BM in chronic rejection.