Ribonucleotide reductase (RNR) is a key enzyme for DNA synthesis since it provides cells with deoxyribonucleotides, the DNA precursors. Class I alpha2beta2 RNRs contain a dinuclear iron center and an essential tyrosyl radical in the beta2 component (protein R2). This is also true for the purified protein R2 of Mycobacterium tuberculosis RNR, as shown by iron analysis, light absorption and EPR spectroscopy. EPR spectroscopy at 286 GHz revealed a high g(x) value, suggesting that the radical is not hydrogen bonded, as in other prokaryotic R2s and in contrast with eukaryotic R2s (from Arabidopsis thaliana and mouse). Furthermore, it proved to be very resistant to scavenging by a variety of phenols and thiols and by hydroxyurea, similar to the Escherichia coli radical. By comparison, the plant and mouse radicals are very sensitive to drugs such as resveratrol and 2-thiophenthiol. The radical from M. tuberculosis RNR does not seem to be an appropriate target for new antituberculous agents.