Balb/c mice made chimaeric by neonatal injection of semi-allogeneic (A/J x Balb/c) F1 hybrid spleen cells develop anti-DNA and rheumatoid factor-like antibodies in the context of hypergammaglobulinaemia with marked elevation of IgG1 and IgE serum levels. Chimaeric mice also display increased levels of antibodies to different haptens and to tobacco mosaic virus (TMV). The allotypic marker of the A/J strain is present on anti-DNA and anti-hapten antibodies. In addition, spleen cells of chimaeric mice spontaneously produce high levels of IgG1 and anti-DNA antibodies in vitro and this hyperactivity is abolished after lysis of donor lymphocytes. These findings indicate that polyclonal activation of donor B cells plays an important role in this model of autoimmunity.