HIV-1 Nef is one of the accessory proteins synthesized in the early stage of AIDS virus reproduction and is abundantly found in infected cells. The name derives from its negative factor thought at the beginning but presently it is remarked as the protein which bears a most distinctive biological characteristic of AIDS virus. The protein interacts directly with the signal transduction protein of the host T cell and works effectively on AIDS infection or on long term survival of the infected cells or induces apoptosis of non-infected cells. It is also involved in the endocytosis and degradation of receptor protein of the cell surface such as CD4 and MH4, important for AIDS virus infection. The product is prepared by immunizing rabbit with recombinant Nef protein which was over-expressed in E. coli with a plasmid carrying the Nef coding region of HIV-1 virus, subtype B and highly purified by several steps of chromatography. Using this antiserum in Western blotting, the band of 27 kD corresponding to HIV-1 Nef was observed in the extract of the AIDS virus infected cells.