In an attempt to clarify the participations of cellular immunity in the development of tubulo-interstitial lesions, aberrant expressions of major histocompatibility complex (MHC) class II antigens and Ki-67 nuclear antigen on the renal tubular epithelial cells were studied. Ki-67 antigen was known to appear in all phases of cell cycle except for Go. Nine normal kidney specimens (4 males and 5 females) and 117 kidney specimens obtained from patients with kidney diseases (54 males and 63 females) were examined with the indirect immunofluorescence technique using murine monoclonal antibodies against HLA-DR (lal), HLA-DQ (Leu10) and Ki-67 nuclear antigen. Patients included 100 with glomerular diseases, and 16 with tubulo-interstitial diseases consisting of 4 acute tubular necrosis (ATN), 7 acute tubulo-interstitial nephritis (AIN), one renal allograft rejection and 4 sarcoidosis. In normal kidney, HLA-DR was solely noted in only two specimens (22.2%) at the basal portion of proximal tubular epithelial cells. In tubulo-interstitial diseases 11 (68.8%) out of 16 patients showed diffuse and intense expressions of HLA-DR concomitant with HLA-DQ in 6 of 13 (42.9%), and 11 of 13 (84.6%) were positive for Ki-67 nuclear antigen. Especially, in AIN and allograft rejection, intense expression of HLA-DR, DQ and Ki-67 nuclear antigen were observed in 100%, 86%, 100%, respectively. In ATN 3(75%) were positive for HLA-DR and Ki-67, but not for HLA-DQ. In contrast, only 12(15.6%). 2(2.6%) and 2(4.8%) of primary glomerular disease were weakly positive for HLA-DR, DQ and Ki-67 nuclear antigen, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)