A subpopulation of peripheral blood lymphocytes with the ability to lyse target cells coated with specific antibody (antibody-dependent cell-mediated cytotoxicity, ADCC) was serially studied in a patient with hepatitis B antigen-associated periarteritis nodosa. The effector lymphocytes possess FC and complement receptors but do not require complement for functional activity. We found that the patient's ADCC was decreased during periods of disease activity and was almost normal during remission. The patient's serum could block ADCC in normal lymphocytes, and the blocking ability correlated with the concentration of immune complexes as determined by the Raji cell assay (a radioimmunoassay using complement receptors on human cultured lymphoblastoid cells). The concentration of immune complexes and the ADCC blocking ability of ther serum both correlated with disease activity. Serum from five other patients with active vasculitis was found to contain significant amounts of immune complexes and was able to block normal ADCC. It appears that the ADCC assay can be used to detect the presence of circulating immune complexes and to monitor disease activity in periarteritis nodosa.