Neutral endopeptidase (EC 188.8.131.52; NEP), originally isolated from renal tubular brush border, is a cell surface peptidase identical to the CD 10 antigen (or CALLA; common acute lymphoblastic leukemia antigen) in lymphoid cells. We studied the serum NEP levels daily after transplantation (Tx) in 19 renal allograft recipients. The NEP activity was determined with a two-step enzymatic assay utilizing a fluorogenic substrate (Suc-Ala-Ala-Phe-AMC; see text) and related to clinical signs of graft rejection, to signs of immunoactivation in transplant fine-needle aspiration biopsy (FNAB) specimens, to renal function, and to serum levels of C-reactive protein. The serum NEP levels remained normal (peak level 10.3 ± 1.8 µg/l on days 6-9 after Tx, initial level after Tx 7.3 ± 1.4 µg/l on day 2; mean values ± SEM) in patients who neither showed clinical signs of rejection nor had findings of immunoactivation in FNAB samples. On the contrary, the serum NEP levels rose clearly in patients developing acute rejection verified clinically and in FNAB samples (peak value 90.4 ± 18.7 µg/l on days 6-9 post-Tx; p < 0.001 compared with patients without signs of immunoactivation) and even in patients having immunoactivation in FNAB without clinical evidence of rejection (108.2 ± 22.4 µg/l, p < 0.001). Serum NEP peak appeared 2-3 days before clinical diagnosis of rejection and a positive finding in FNAB samples. Serum NEP increments did not correlate with changes in serum creatinine, delayed onset of renal excretory function, blood leukocyte count, C-reactive protein level, or infections. Thus, the serum NEP activity was shown to increase after renal allotransplantation associated with early phases of immunoactivation and development of acute graft rejection. Because of the limited number of patients studied, the clinical implications of these preliminary observations for kidney transplant monitoring clearly need confirmation in larger studies.