Leptospirosis is an infectious bacterial disease caused by Leptospira species. In this study, we cloned and se- quenced the gene encoding the immunodominant protein GroEL from L. interrogans serovar Autumnalis strain N2, which was isolated from the urine of a patient during an outbreak of leptospirosis in Chennai, India. This groEL gene encodes a protein of 60 kDa with a high degree of homology (99% similarity) to those of other leptospiral serovars. Recombinant GroEL was overexpressed in Escherichia coli. Immunoblot analysis indi- cated that the sera from confirmed leptospirosis patients showed strong reactivity with the recombinant GroEL while no reactivity was observed with the sera from seronegative control patient. In addition, the 3D structure of GroEL was constructed using chaperonin complex cpn60 from Thermus thermophilus as template and vali- dated. The results indicated a Z-score of -8.35, which is in good agreement with the expected value for a pro- tein. The superposition of the Cα traces of cpn60 structure and predicted structure of leptospiral GroEL indi- cates good agreement of secondary structure elements with an RMSD value of 1.5 . Further study is necessary to evaluate GroEL for serological diagnosis of leptospirosis and for its potential as a vaccine component. Leptospirosis is an infectious bacterial disease caused by Leptospira species. In this study, we cloned and se- quenced the gene encoding the immunodominant protein GroEL from L. interrogans serovar Autumnalis strain N2, which was isolated from the urine of a patient during an outbreak of leptospirosis in Chennai, India. This groEL gene encodes a protein of 60 kDa with a high degree of homology (99% similarity) to those of other leptospiral serovars. Recombinant GroEL was overexpressed in Escherichia coli. Immunoblot analysis indi- cated that the sera from confirmed leptospirosis patients showed strong reactivity with the recombinant GroEL while no reactivity was observed with the sera from seronegative control patient. In addition, the 3D structure of GroEL was constructed using chaperonin complex cpn60 from Thermus thermophilus as template and vali- dated. The results indicated a Z-score of -8.35, which is in good agreement with the expected value for a pro- tein. The superposition o f the Cα traces of cpn60 structure and predicted structure of leptospiral GroEL indi cates good agreement of secondary structure elements with an RMSD value of 1.5 . Further study is necessary to evaluate GroEL for serological diagnosis of leptospirosis and for its potential as a vaccine component .