Isoniazid resistance in Mycobacterium tuberculosis (MBT) is associated with point mutations in codon 315 of the katG gene. Two PCR protocols were developed for detection of point mutations in codon 315. Most frequent point mutations (AGC ACC and AGC AGA) were identified in codon 315 by using two sets of primers, either of which included an additional competitive blocking primer with a 3'-terminal phosphate group in order to prevent nonspecific amplification. Polymerase chain reaction with a set of two primers, one of which contained five locked monomers (LNA), permits one to identify any of six known mutations in codon 315 of katG and, thereby, discriminate between isoniazid-sensitive and resistant MBT isolates. The structure and purity of the 17-nt long LNA-containing oligonucleotides were characterized by MALDI-TOF mass spectrometry; and the duplex formed by two 17-nt long LNA-containing complementary oligonucleotides was analyzed by thermal denaturation.