One of the mechanisms of isoniazid resistance toMycobacterium tuberculosishas been proved to be the chromosomal deletion ofkatG. Based on this finding, 22 isoniazid-resistant isolates ofM. tuberculosisobtained in Japan and Yemen were analysed forkatGby polymerase chain reaction and catalase activity. Only six (27%) of the 22 isolates were compatible with the mechanism (lack of amplification ofkatGand loss of catalase activity). In contrast, eight isolates (36%) werekatGpositive but catalase activity-negative and eight (36%) were positive for both factors, indicating that isoniazid resistance is multifactorial and the deletion ofkatGwas not the major cause of resistance in the isolates examined in this study.