[en] A solid phase radioimmunoassay (RIA) to study antibody response in leprosy was set up with natural trisaccharide-octyl-bovine serum albumin (NT-O-BSA), a Mycobacterium leprae (M. leprae)-specific synthetic antigen. Both IgM and all immunoglobulin (Igs) antibodies in sera of 62 lepromatous (28 new and 34 old LL/BL), 18 tuberculoid (TT/BT), 25 tuberculosis (TB) patients and 33 normal controls, were studied for their reactivities to the antigen using ¹²⁵I-labelled anti-human IgM and anti-human Igs, respectively. Binding activities and positivity rates (based on X+3SD of normal controls) of IgM antibodies were the highest in new LL/BL and moderate in TT/BT sera. Tuberculous and normal sera were devoid of IgM antibodies to the antigen. The same pattern of binding activities and positivity rates were also found for all Igs to the NT-O-BSA. However, positivity rates of all Igs were higher than those of IgM, particularly in sera of LL/BL and TT/BT. Only one out of fourteen TB and no normal sera were positive. Our results suggest that the RIA with this synthetic antigen is highly sensitive and specific to M. leprae injection and should be useful for serodiagnosis and also seroepidemiology of leprosy. (author). 10 refs, 1 fig., 1 tab