Sera from 92 patients were tested by the ELISA method for the presence of IgM antibodies to phenolic glycolipid-1 (PGL-1) of Mycobacterium leprae, and of both IgM and IgG antibodies to the whole M. leprae bacillus. All untreated lepromatous patients exhibited high antibody levels in all three assays. A sharp decline of IgM antibodies to PGL-1 and whole M. leprae was observed during the first two years of therapy, while IgG antibodies to whole M. leprae showed a progressive decrease only over a number of years. Low titers of IgM antibodies to PGL-1 and IgG antibodies to whole M. leprae could be detected in about 50% and 75% of patients, respectively, after more than ten years of treatment, with only 15% showing persisting IgM antibodies to the whole bacillus. Antibody levels as measured by the three assays used were correlated with the bacterial index in patients treated for less than four years. In patients treated longer than four years, only IgM antibodies, whether directed to PGL-1 or to whole M. leprae, remained correlated to the bacillary load. Tuberculoid patients exhibited a different antibody pattern, showing a lower frequency (and lower levels) of antibodies of PGL-1 and of IgG antibodies to whole M. leprae than lepromatous patients, and no detectable IgM antibodies to the whole bacillus. IgG antibodies to whole M. leprae were more frequently noted than antibodies to PGL-1, the latter declining more rapidly during therapy.