In an attempt to demonstrate the presence of antibodies to ATL-associated type-C virus particles (ATLV), an indirect immunoferritin method of immunoelectron microscopy was performed on an ATLV-producing human cord T-cell line (MT-2) and short-term cultures of ATL cells. All five sera from ATL patients seropositive to ATL-associated antigens (ATLA) but not three sera from healthy adults were positive for the ferritin labeling of ATLV and plasma membranes in both MT-2 and short-term cultured ATL cells. Sera absorbed with sheep red blood cells or human T-cell acute lymphatic leukemia cells labeled ATLV much more intensely with ferritin than with plasma membranes. These results demonstrated at the ultrastructural level that anti-ATLA-positive sera contained antibodies to surface glycoproteins and/or structural proteins of ATLV and that they differed from anti-Forssman or anti-T-cell antibodies. It was also demonstrated that anti-ATLA-positive sera showed the same reactivity with ATLV from both MT-2 and short-term cultured ATL-cells by the immunoferritin method.