Vesicular stomatitis virus (VSV) grown in mouse embryo cells pre-infected with murine sarcoma virus or in chicken cells pre-infected with avian myeloblastosis virus contains, in contrast to virus grown in corresponding control cells, a proportion of virus resistant to antiserum against VSV. Infectivity of this virus fraction can specifically be neutralized with antiserum against murine leukaemia virus (MLV) and avian myeloblastosis virus (AMV), respectively. In addition to neutralization specificity, the particles resistant to anti-VSV serum show also host-range and interference specificity corresponding to the leukosis virus involved. These viruses do not breed true as recombinants, but virus isolated from their plaques is indistinguishable from original VSV. All these facts are consistent with the explanation that there are VSV(MLV) and VSV(AMV) pseudotype particles.