Eighty sera from tuberculosis (TB) patients, 16 Indian and 10 American control sera were analyzed by ELISA for relative titres of antibody against mycobacterial antigens. Levels of specific antibody and mycobacterial Ag in circulating immune complexes (CIC) isolated from these sera were also studied. All these parameters were found to be elevated in TB sera as compared to control sera. Maximum increase was however noted in CIC specific antibody titres. A good correlation was observed between serum and CIC levels of specific antibody (r = 0.72) and between specific antigen (Ag) and antibody (Ab) levels within CIC (r = 0.64). In a few of the TB sera examined, CIC specific Ab contributed less than 1% to the Ab titres in sera. In order to examine the differences between different subgroups within TB patients, a statistical analysis of variance was performed. Sex of the patients had no effect on any parameter. Sputum-positive patients had significantly higher levels of CIC Ag and Ab than the sputum-negative patients, although no significant difference occurred in respect to serum Ab. All three parameters were significantly higher in patients on chemotherapy as compared to fresh untreated cases. The relevance of these observations to the development of a CIC-based immunodiagnostic assay for TB is discussed.