HTLV‐producing T‐cell lines induce cell fusion when cocultivated with a wide variety of indicator cells, suggesting that HTLV envelope antigens interact with the membranes of many cell types. Serum antibodies from adult T‐cell lymphoma‐leukemia (ATL) patients inhibited the formation of syncytia, and sera from British, American and Japanese ATL patients inhibited cell fusion induced by American and Japanese HTLV isolates equally well. No serological cross‐inhibition of syncytium induction was found between HTLV and bovine leukosis virus, Moloney murine leukemia virus and simian sarcoma‐associated virus. A simple syncytium inhibition test in microtiter plates has been developed to provide a rapid screen for antibodies presumed to be specific to HTLV envelope glycoproteins.