Chronic rejection is a major problem in contemporary kidney transplantation. The purpose of this study was to determine whether renal cells are repopulated by extra-renal cells over time or whether the graft remains permanently allogenic. We studied nine explanted allografted kidneys of sex-mismatched donors by means of non-isotopic in situ hybridization (NISH). We used biotinylated centromer-specific DNA probes of the human chromosomes Y and X. In a further step, monoclonal and polyclonal antibodies against CD45, CD3, CD20, CD31, CD1a, S100, alpha-actin, factor Vill and UEA were used to analyse the various infiltrating cell types and the cells involved in allograft arteriopathy. In several cases NISH and immunohistochemistry were combined to facilitate the typing of cells. Our study showed that up to several years after transplantation the glomerular, tubular and endothelial cells retained donor origin. The only cells of recipient origin were the inflammatory cells, predominantly macrophages and T lymphocytes.