Immune complexes were isolated from sera of tuberculosis patients by precipitation with 2.5% polyethylene glycol. The precipitates were characterized by quantitative determination of different immunoglobulin classes by single radioimmunodiffusion, sodium dodecyl sulfate polyacrylamide gel electrophoresis for presence of serum components, Ouchterlony's double diffusion method for detection of complement components C3 and C4, and immuno-dot assay for detection of Mycobacterium tuberculosis antigens. The results showed that polyethylene glycol precipitates of patients' sera were indeed immune complexes, as they contained immunoglobulins, albumin, complement components, and mycobacterial antigens, whereas precipitates from control sera contained mainly albumin. The antibodies present in immune complexes were specific to M. tuberculosis antigen and showed no binding to Escherichia coli antigens. Immune complex levels, as determined by the ability to bind M. tuberculosis antigens in an enzyme-linked immunosorbent assay, were significantly higher in tuberculosis patients (n = 22) than in healthy control subjects (n = 18). Thus, immune complex level could be a useful parameter in the diagnosis of active tuberculosis.