Granzymes A and B are serine proteinases which are stored in the granules of activated cytotoxic T cells and NK cells. Expression of these granzymes by cytotoxic cells in tissues can be used as an activation marker for these cells. To investigate a possible role of cytotoxic lymphocytes in rheumatoid arthritis (RA) and osteoarthritis (OA), we assessed the expression of granzymes A and B by cytotoxic lymphocytes in synovial biopsies from five RA and five OA patients using mAb specific for these serine proteinases. In three of the five RA patients but also in two of the five OA patients granzyme A- and B-expressing lymphocytes were observed in the synovium. Double-labeling immunohistochemical techniques revealed that up to 75% of the granzyme-positive synovial lymphocytes had the CD16+ or CD56+ natural killer cell phenotype. Less than 5% were CD3+, CD8+ cytotoxic T cells, whereas in some patients the phenotype of up to 50% of these cells could not be identified. The presence of granzymes A and B in the synovium of both RA as well as OA patients was confirmed on the molecular level in a second group of 11 RA and 5 OA patients using the polymerase chain reaction. Thus, expression of granzymes A and B occurs in the synovium in patients with RA as well as those with OA. These proteins are mainly expressed by NK cells that may therefore play a role in the pathogenesis of these diseases.