Cell entry of the pandemic virus SARS-CoV-2 is mediated by its spike protein S. As main antigenic determinant, S protein is in focus of various therapeutic strategies . Besides particle-cell fusion, S mediates fusion between infected and uninfected cells resulting in syncytia formation . Here we present sensitive assay systems with a high dynamic range and high signal-to-noise ratios covering not only particle-cell and cell-cell fusion, but also fusion-from-without (FFWO). In FFWO, S-containing viral particles induce syncytia independently of de novo synthesis of S. Neutralizing antibodies as well as sera from convalescent patients inhibited particle-cell fusion with high efficiency . Cell-cell fusion, in contrast, was only moderately inhibited despite requiring levels of S protein below the detection limit of flow cytometry and Western blot . The data indicate that syncytia formation as pathological consequence during Covid-19 can proceed at low levels of S protein and may not be effectively prevented by antibodies.