Cell-mediated immune (CMI) responses were measured in 39 lepromatous, 44 erythema nodosum leprosum (ENL), and 22 post-ENL patients. The leukocyte migration inhibition test was used to measure CMI responses to mitogen phytohemagglutinin-P (PHA), crossreacting antigen purified protein derivative (PPD) of tuberculin, and armadillo-derived whole and sonicate Mycobacterium leprae. "Early T" lymphocytes of the peripheral blood were also enumerated using the rosetting technique. Significantly enhanced immune responses (lower migratory indices) were found to whole M. leprae during ENL. Although responses were high with PHA and PPD, they were identical in all of the groups, indicating that during ENL reaction M. leprae-specific responses are enhanced. "Early T" lymphocytes also showed a significant increase in ENL reactions compared to lepromatous patients. However, there was no response to the leprolin skin test in ENL patients in contrast to the enhanced in vitro CMI responses.