HIV-1 Gag p24 is a capsid protein that constitutes the core of AIDS virus HIV-1 and is produced by the digestion of its precursor Gag p55 by HIV-1 protease. This protein is indispensable to the reproduction of AIDS virus and constitutes an essential element for the AIDS virus particle construction. As this protein is detectable from the early stage of AIDS virus infection, it is used as a marker for observation of the development in the patient's condition after treatment, as it indicated the amount of virus in the blood. The product is prepared by immunizing rabbit with recombinant p24 protein which was over-expressed in E. coli with a plasmid carrying the Gag p24 coding region of HIV-1 virus, subtype B, and highly purified by several steps of chromatography.