The presence of circulating Immune complexes (CIC) in sera from patients with pulmonary tuberculosis was demonstrated by 3 techniques (a) latex agglutination (b) 3.5% PEG precipitation and determination of optical density (O.D.) at 280 nm and (c) radioimmunoassay (RIA) of CIC using bovine spermatozoa. 40 normal control sera and 100 T.B. patients sera were included in the study. 12% cases were positive for CIC by all the 3 methods mentioned above, 13% were negative by all the 3 methods and the remaining patients were positive by one or more methods of detection. To correlate the levels of CIC as detected by different techniques with the activity of the disease, patients were broadly grouped as (a) sputum positive and (b) sputum negative. Higher levels of CIC were obtained in sputum positive cases than sputum negative by all the 3 methods studied. While IgG, IgA and IgM were elevated in the CIC of T.B. patients, and IgG and IgA were also present in controls, IgM immunoglobulins were detected only in patients and not in controls. The effect of antitubercular treatment on the levels of CIC was also evaluated and it was found that the levels of CIC remained unchanged even after prolonged chemotherapy.