LDH isoenzymes were studied in tissue extracts of 78 cases of leprosy. All 25 control tissues showed five LDH isoenzymes corresponding to those of human sera. All tissues from the leprosy cases showed five similar bands. Seventeen cases showed additional LDH isoenzymes (anomalous bands). In 12 cases there was a single extra band with an eF value of either 0.125 +/- 0.015 or 0.525 +/- 0.015, and five cases showed both these bands. Additional bands were observed only in cases positive for acid-fast microorganisms (17 of 27 cases), and their presence correlated well with bacterial load (as judged by the BI) and viable organisms (as judged by the MI). Four cases with a high BI and MI did not show anomalous bands, however. A plausible explanation for these bands is that they originate from viable M. leprae.