Lepromin, a product containing Mycobacterium leprae from patients' tissues, fails to elicit skin reactions in lepromatous patients; this non-reactivity was utilized as a means of identifying the mycobacterium isolated in mouse foot-pads from leprosy patients. To do this a suspension of acid-fast bacilli prepared from mouse foot-pads infected with a typical isolate was compared to human lepromin. The isolate was in fourth passage, and the multiplication since first isolation had diluted the original inoculum to insignificant levels. Three preparations were tested: A, A foot-pad preparation containing 1.6 x 107 organisms/ml, B, lepromin diluted to the same bacillary content, and C, undiluted lepromin containing 4.2 x 107 organisms/ml.

The reactions were compared in 34 lepromatous and 30 tuberculoid patients. All 34 lepromatous patients were negative in both early (Fernandez) and late (Mitsuda) reactions to all 3 antigens. In the tuberculoid patients the late reactions were positive in 66.7, 70.0 and 90.0 per cent to antigens A, B, and C, respectively. The size of the reactions to A and B were closely correlated in the individual patients.

Experience in leprosy patients with suspensions prepared from 16 other mycobacterial cultures is reported or reviewed. None of them produced reactions that correlated with those of lepromin. Thus the lack of reactivity to lepromin of lepromatous patients appears to be a specific phenomenon.

The results provide evidence that the foot-pad isolates and M. leprae are immunologically identical.

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