Sera from tuberculous and leprous patients have been examined for antibody reactivities against components of BCG sonicate (BCGS) antigen. A crossed immunoelectrophoresis with intermediate gel reference system was used in which more than 40 components of BCGS could be identified. Forty (74.1%) out of 54 tuberculous sera and 68 (90.7%) out of 75 leprous sera reacted with at least 1 component of BCGS. While tuberculous sera reacted with 9 distinct components of BCGS, leprous sera reacted with at least 12. Components of BCGS precipitated by tuberculous sera were not specific as they were also precipitated by leprous sera. Overall, non-specific antibody responses were found to be dominant among tuberculous sera and by comparison, the reactivity of leprous sera with BCGS components was of a higher magnitude. Among tuberculous sera, precipitating activity was maximal among those taken from chronic treated cases with relapse followed by those obtained from treated and untreated new cases. Some components of BCGS to which both tuberculous and leprous sera showed strong reactivity have been characterized. It is concluded that immunoprecipitation methods with BCG derived antigens are not useful for the detection of a specific antibody response in tuberculosis or for discrimination between tuberculosis and leprosy.