Acute humoral rejection (AHR) occurs in 20–30 % of acute renal rejection cases and has a poorer prognosis than cellular rejection .AHR is mediated by anti HLA class I or class II donor specific antibodies (DSA Ig G found in patient serum). Detection of these DSA can be done by performing flow cytometry cross match (FCXM) between patient serum and donor lymphocytes. 40 years old female patient with chronic renal failure received kidney graft from her husband. Pretransplant complement dependent microlymphocytotoxicity CDC crossmatch was negative and FCXM was T negative / B positive. Ten days after transplantation serum creatinine level rose up suddenly to 210 µ mol/L. Renal biopsy has revealed features of acute vascular rejection. FCXM between patient two serum samples (one before transplantation and the second during rejection episode) and donor lymphocyte showed increased actual double level of DSA in the second sample compared to the first one. She received 14 sessions of plasma-exchange. Second FCXM done after the end of plasma-exchange showed decreased level of DSA, this was accompanied with drop of serum creatinine to normal level (100 µ mol/L). FCXM can be prospectively used in diagnosis of AHR and monitoring the response to its treatment.