To investigate the characteristics of interstitial inflammatory cells and possible involvement of nudelta T cells, 16 renal allograft biopsies showing chronic rejection were stained by immunohistochemical method and correlated with the data of peripheral blood evaluated by flow cytometry. For immunophenotyping, fresh frozen sections were stained with monoclonal antibodies against CD3, CD4, CD8, CD68, CD56, TCRdelta1 and HLA DR. Paraffin embedded tissue was stained with CD45RO, CD20-Cy and CD68. Nine cases of nonspecific tubulointerstitial change and 4 cases of nonallograft tubulointerstitial nephritis were used as a control. Inflammatory infiltration was present in all cases studied. T cells predominated in the interstitium of chronic rejection and were followed by macrophages and B cells. The degree of interstitial infiltration of frozen section was not accordant with that of paraffin sections. Allografts with nonspecific tubulointerstitial changes or tubulointerstitial nephritis of native kidneys showed similar distribution pattern in terms of type and degree. However, the degree of infiltrate did not give any statistical significance among groups. The CD4/CD8 ratios in interstitial infiltrates were less than 1.0 in 6 cases and was not accordant with those of peripheral blood. Proportion of nudelta T cells increased over 10% in 2 cases in tissue and in 3 cases in peripheral blood. In 3 cases of chronic rejection in which both tissue and blood results were available, there was no concordance of CD4/CD8 or nudeltaT/CD3 between them. Tubular expression of HLA DR was, however, present only in 4 cases of chronic rejection. In conclusion, T lymphocytes were predominant regardless of diagnosis or disease activity. T lymphocyte subset did not give any suggestion as to the diagnosis or disease activity in chronic rejection. Furthermore nudelta T cells had only limited value. Lymphocytic subsets in peripheral blood would not be predictors of tissue destruction in chronic rejection.