BACKGROUND: The 463 codon mutation of katG gene has been reported as an useful marker for the detection of isoniazid(INH) resistant strains of M. tuberculosis. This study aimed to elucidate relationship between 463 mutation in katG gene and INH resistance in M. tuberculosis. METHOD: DNA was extracted from 28 INH susceptible strains(MIC > or = 0.2microg/ml on the Lowenstein Jensen media) and used for amplification of 189bp fragment containing 463 codon by PCR. Amplified fragments were digested by restriction enzyme Msp I, analyzed by single strand conformation polymorphism(SSCP) in the MDE gel and sequenced to prove mutation. RESULT: Only 7(25%) out of 28 were digestible by restriction enzyme Msp I. The SSCP pattern of 21 strains were distinctly different from that of M. tuberculosis H37Rv. Msp I undigestible PCR fragment was substituted at 463 codon from Arg(CGG) to Leu(CTG). CONCLUSION: This finding clearly indicate no relationship between 463 codon mutation of the katG gene and INH resistance.