Abstract: Objective To investigate the efficacy and safety of half-dose Zenapax for prevention of acute rejection after renal transplantation.Methods According to the immunosuppressive regimen and renal function after transplantation,patients were divided into 4 groups,namely groups A,B,C,and D of 90,73,11 and 13 patients,respectively.Blood creatinine measured 1 week after operation was <176.6 μmol/L in groups A and B,and was >353 μmol/L in groups C and D.Patients in groups A and C were given 25 mg Zenapax(0.5 mg/kg)and MMF 0.75 g before operation,and those in groups B and D had only MMF of 0.75 g.All patients were given Pred,CsA and MMF after operation,and the rejection episodes,the time of acute rejection onset,the rate of rejection reversal and complications were analyzed in the time period of 6 months after operation.Results After the operation,13 patients(14.4%)developed acute rejection in group A,18(24.6%)in group B,6(54.5%)in group C and 7(53.8%)in group D(P<0.01).The incidence of acute rejection in group B was significantly lower than that in groups C and D groups(P<0.01),and the latter two groups had similar incidence.The time of acute rejection onset ranged from 3 to 9 days postoperatively(mean 6.2±3.2 days)in group A,significantly delayed as compared with that in group B(range 2-8 days,mean 4.7±3.1days),group C(range 2-7 days,mean 4.3±4.2 days)and group D group(range 2-9 days,mean 3.9±3.5 days),but the time was similar between groups B,C,and D(P>0.05).All acute rejection cases in group A was reversed,and the rate of reversal was 88.9%(16/18)in group B,83.3% in group C,and 71.4% in group D.No significant differences were noted in such complications as infection,vascular injuries or gastrointestinal reactions between the 4 groups(P>0.05).Conclusion Zenapax at the dose of 25 mg can safely decrease the risk of acute rejection in patients with good postoperative renal function recovery,but dose not seem effective in patients with delayed graft function recovery.