Mycobacterium tuberculosis isolates were studied by comparing large restriction fragment (LRF) patterns produced by digestion of chromosomal DNA with infrequent-cutting endonucleases and pulsed-field gel electrophoresis. Four cultures of H37Rv and 36 clinical isolates of M. tuberculosis were compared by using DraI, AsnI, XbaI, and SpeI. DraI and AsnI allowed easy visual separation of 18 of 21 epidemiologically unrelated strains. XbaI and SpeI allowed discrimination of all 21 unrelated strains, including the 3 strains inseparable with DraI and AsnI, but comparison of LRF patterns was more tedious because of overlapping fragments. A total of 26 isolates belonging to 10 clusters of related isolates were compared by pulsed-field gel electrophoresis, with all related isolates giving identical LRF patterns. These included multiple isolates from the same patient or the same family. The same grouping of clustered isolates was obtained when BamHI DNA digests were hybridized with two probes from the insertion sequence IS6110. Long-term laboratory passage of H37Rv produced minimal detectable changes in LRF patterns. LRF patterns are useful tools for epidemiologic studies of tuberculosis without the need for radioactive or specific DNA probes.