Tuberculosis remains the greatest cause of death worldwide due to a single pathogen. In order to identify the genes required for the pathogenicity of Mycobacterium tuberculosis, a functional genomic approach was developed. A library of signature-tagged transposon mutants of this bacterium was constructed and screened for those affected in their multiplication within the lungs of mice. From 1927 mutants tested, 16 were attenuated for their virulence. The insertions harboured by the selected mutants were mapped on the M. tuberculosis genome and most of the mutated loci appeared to be involved in lipid metabolism or transport across the membrane. Four independent mutations identified a cluster of virulence genes located on a 50 kb chromosomal region. These genes might be involved in the production of phthiocerol and phenolphthiocerol derivatives, a group of molecules restricted to eight mycobacterial species, seven of them being either strict or opportunistic pathogens. The interaction of five mutant strains with mouse bone marrow macrophages was investigated. These five mutants were still able to multiply in this cell type. However, in three cases, there was a growth defect in comparison with the wild-type strain. The other two strains exhibited no clear difference from the virulent strain, MT103, in this model. This study, which is the first global research of virulence factors of M. tuberculosis, opens the way to a better understanding of the molecules that are key players in the interaction of this pathogen with its host.