n the life cycle of enveloped viruses, assembly of new infectious virus particles involves interactions between viral proteins, nucleic acids and lipids, as well as between proteins of viral and host cellular origins. Although the majority of host cell-surface proteins are excluded when enveloped viruses are released by budding through cell membranes, the incorporation of host proteins into the nascent virion is a frequent phenomenon. This is because the host cellular machinery, particularly elements of the cytoplasmic or nuclear membrane, are actively participating during assembly of the newly formed viral entity. For example, previous studies have shown that avian myeloblastosis virus acquires membrane ATPase (Ref. 1); H-2 antigens are incorporated by murine leukaemia virus 2 and Friend virus particles 3 ; and HLA-A and HLA-B antigens are found embedded in the envelope of feline leukaemia virus 4 . In addition, it has been shown that cellular histones, Thy-1, and receptors for human interleukin 2 are selectively incorporated in simian virus 40 (Ref. 5), murine leukaemia virus 6 , and human T-cell leukaemia virus type 1 (HTLV-1) 7 , respectively. Finally, other reports have indicated that human CD4 is assembled into avian leukosis virus particles 8 ; major histocompatibility complex (MHC) class II molecules are physically present on the surface of EpsteinÐBarr virus 9 ; hostencoded annexin II is endogenously associated with human cytomegalovirus (HCMV) 10 ; and complement control proteins CD55 and CD59 are linked with the external membrane of HTLV-1 and HCMV (Ref. 11).