A T-cell line, MT-2, derived from human cord blood lymphocytes by cocultivation with adult T-cell leukemia (ATL) cells is a continuous producer of type-C virus particles. Electron microscopy of MT-2 cells cultured for 1-3 weeks in medium containing 10% ATL patients' sera revealed agglutination of type-C virus particles within the electron-dense deposits in the extracellular spaces. No such agglutination occurred in control cultures supplemented with normal human or fetal calf serum. These results provide direct evidence for the specific reactivity of ATL patient's sera with type-C virus particles in the MT-2 cell line at the ultrastructural level.