Fifty-six male and 14 female leprosy patients, aged 11-62, were studied for cell-mediated immunity (CMI) and histocompatibility antigens. Healthy blood donors were used as normal controls. All patients were receiving antileprosy drugs. T and B cells were detected by E and EAC rosette formation technics, and the leukocyte migration test (LMT) was done in the presence of PHA. HLA antigens were defined by a modified N.I.H. lymphocytotoxicity test in order to type 48 patients and 100 controls. There was a significant difference (p less than 0.01) in the number of T cells between tuberculoid and lepromatous forms of the disease as compared to normal controls. We did not observe any differences in EAC rosette cells. It should be noted that the migration index is significantly higher in controls than in leprosy patients for PHA. There are no significant differences in the distribution of the A locus antigens between leprosy patients and controls, although a higher percentage of A-11 was obtained in leprosy patients. A slight elevation of B5 antigen was observed but these results are preliminary and our information regarding the B locus is incomplete. Thus, it is difficult to establish any precise relationship between HLA antigen and leprosy at this stage.