Cathepsin G is a serine protease located in the azurophil granules of neutrophils. We have shown previously that cathepsin G stimulates human lymphocytes. In this study, we demonstrate that cathepsin G exhibits specific, saturable and reversible binding to lymphocytes: B cells, CD4+ T cells, CD8+ T cells, and natural killer (NK) cells. Phenylmethylsulfonyl fluoride (PMSF) -inhibited cathepsin G also bound to lymphocytes, although both the number of binding sites and the affinity were less than those of native cathepsin G. The binding of cathepsin G to lymphocytes showed cooperativity, but that of PMSF-inhibited cathepsin G did not. PMSF-inhibited cathepsin G was able to partially displace bound cathepsin G. These results suggest that molecules on the lymphocyte surface that bind cathepsin G recognize not only the active site of cathepsin G but also other sites of this molecule, but the active site has an important function in this binding because stimulation of lymphocytes requires the binding of proteolytically active cathepsin G.