The importance of antibodies for elimination of polyomavirus infection and the prevention of virus induced oncogenesis was studied, X-linked immunodeficiency (XID) mice, IgM-/- single knockout and IgMI-/- CD8-/- double knockout mice, all defective in antibody production, and normal control mice were infected with polyomavirus as adults. The mice were followed for presence of polyoma DNA with a polyoma specific polymerase chain reaction (PCR) over 6 weeks post infection (p.i.), a time point at which polyomavirus DNA is no longer detected in normal adult infected mice. As expected, virus DNA was not detected in normal mice 6 weeks p.i. In both IgM-/- single knockout and IgM-/- CD8-/- double knockout mice a disseminated infection was still observed by 6 weeks p.i. and the latter group of mice succumbed around two months p.i. In XID mice, only one third of the mice were still positive for viral DNA 6 weeks p.i. No polyomavirus induced tumors were observed in any of the mice during the 2-4 month observation period.