A sandwich enzyme-linked immunosorbent assay (ELISA) for immune complexes of human T cell leukemia virus type I (HTLV-I) was developed using monoclonal antibody (MoAb) 3G1 which recognizes a different epitope on HTLV-I to that with which natural human anti-HTLV-I antibody binds. The assay was capable of titrating artificial immune complexes not only at antigen-antibody equivalence but also at antibody excess. Although the antigen-antibody ratios could not be determined in the individual sera from patients with overt ATL, the level of immune complexes in three out of four sera was estimated to be 250 +/- 36 ng/ml. Immune complexes of HTLV-I could not be identified in sera obtained from one patient with overt ATL, three healthy HTLV-I carriers and three normal human controls.