Objective To establish the method to distinguish the constitution of MRL/lpr lupus mice and to study the gene and protein expression of renal B cell-activating factor ( BAFF) , B cell stimulating fac-tor receptor ( BAFF-R) , so as to reveal the factors at molecular biological level leading to the differences of constitution.Methods Firstly, MRL/lpr female mice of 12-week age were classified into cold consti-tution, heat constitution and normal constitution.Secondly, 20 mice were randomly selected respectively from cold constitution group, heat constitution group and normal constitution group.Thirdly, every 10 mice in each group were sacrificed at 12-week age and at 20-week age respectively.Then, expressions of renal BAFF mRNA, BAFF-R mRNA, BAFF protein and BAFF-R protein were examined by real-time PCR and Western Blot.Results Expressions of BAFF mRNA ,BAFF protein and BAFF-R protein were the lowest in the cold constitution group of all the groups.The expression of BAFF-R mRNA showed no significant difference among all groups.Conclusion The constitutional differences could be related to the expressions of renal BAFF mRNA ,BAFF protein and BAFF-R protein .