An M. Iprae::pHC79 cosmid genomic library was screened for thernidentification, isolation and characterization of the 34 kd antigen gene ofrnMycobacterium leprae using its homologue, the 34 kd antigen gene ofrnMycobacterium tuberculosis, as a DNA probe. Qut of a total of 79 differentrncosmids screened, covering about 88 % of the genome of M. leprae, one cosmidrndesignated as BL 1527 was found to harbor the gene of interest. Restriction enzymernanalysis of the cosmid BL 1527 yielded four DNA fragments - two containing therncomplete gene and two containing part of the gene. The four DNA segments werernsubcloned in plasmid vectors pUCBM21 and pBKS+ and characterized physicallyrnand immunologically. Physical characterization of the gene by nucleotidernsequencing revealed that the 34 kd antigen gene has an open reading frame of 801rnnucleotides encoding 266 amino acid residues bearing a molecular weight of28,887rndaltons and an isoelectric point of 4.3. Homology comparison with the 34 kdrnantigen gene from M.tuberculosis has revealed 80.6 % (633 nucleotides) identityrnat the DNA level and 85 % (221 amino acids) identity at the protein level.rnImmunological characterization by western blot analysis has shown that the genernis expressed in E. coli from its own promotor signals and that the recombinantrnprotein is recognized by the monoclonal antibody F-126. 2 which is specific for thernmycobacterial 34 kd antigens. Crossed Immuno-electrophoresis analysis hasrnestablished that the recombinant 34 kd antigen corresponds to the non secretedrnAntigen 84 of mycobacteria.