Granzymes A and B are serine-proteinases stored in the granules of activated cytotoxic T-lymphocytes and natural killer (NK) cells. Expression of granzymes in tissues can be used as an activation marker for cytotoxic cells. Using mAbs specific for human granzyme A or B in immunohistochemical staining techniques we investigated expression of granzyme A and B by lymphocytes infiltrating acutely rejected renal allografts. Twelve core needle biopsies were taken from ten different patients during an episode of acute rejection. Eleven biopsies contained high numbers of granzyme A and B positive lymphocytes infiltrating tubular epithelium, and vascular and glomerular structures. In one patient infiltrating lymphocytes did not express granzyme A and only low amounts of granzyme B. No correlation was found between the number of granzyme positive cells and the severity of the rejection as classified by conventional histological criteria. In one tissue specimen from a patient with a renal allograft without signs of rejection, the number of granzyme positive cells was much lower compared to that of the transplant group. In spite of the presence of a marked inflammatory infiltrate, no granzyme positive cells were detected in renal biopsies from patients with various inflammatory, not transplant-related, renal diseases. Phenotypic analysis showed that granzymes A and B were expressed by CD56+ NK cells and CD3+ cells, representing cytotoxic T-lymphocytes. Thus, this study demonstrates that granzyme A and B protein-expressing lymphocytes infiltrate the kidney allografts during an acute cellular rejection but not in several other inflammatory renal diseases.(ABSTRACT TRUNCATED AT 250 WORDS)