We have demonstrated previously that the envelope protein of a murine retrovirus, Moloney murine leukaemia virus, has immunosuppressive properties in vivo. This property was manifested by the ability of the protein, when expressed by tumour cells normally rejected by engrafted mice, to allow the env-expressing cells to escape immune rejection and to proliferate. Here, it is shown that this property is not restricted to the envelope of a murine retrovirus, but is also shared by the envelope encoded by a primate retrovirus, Mason-Pfizer monkey virus.