Printed in Great Britain 3II Immunoglobulins in Human Serum Reactive with Murine

Fresh human sera neutralize Friend leukaemia virus (FLV). The activity is abolished by heating (56 °C for 30 min), hydrazine and EDTA treatment, sugges-ting the involvement of complement. Human sera also contain antibody which specifically reacts with FLV. Following incubation with human whole serum, IgG and F(ab') ~ fragments, FLV acquire the ability to bind anti-human Ig sera, as shown in neutralization a d radioimmunoprecipitation assays. The same is true for FLV-infected cells but not for uninfected control ce ls, as assessed by immuno-fluorescence. The binding of antibody to FLV-infected cells is prevented by pre-treating the cells with antisera to FLV and FLV-gp7I, and is competed by FLV, FLV-gp7I and, to a lesser extent, by FLV-pI2 and p3o