Abstract: A novel cell line chronically infected with HIV-2.sub.NIHZ virus secretes the precursor of the viral envelope proteins in extracellular medium. The cells grow well in serum-free medium which enables the concentration of the conditioned medium of the cells to be greatly increased (>100 fold). Such a concentration step is otherwise impossible to achieve if the cells are grown under conventional conditions using 10% fetal calf serum. A specific affinity procedure is provided to purify the gp160 protein from the concentrated conditioned medium of cultures of 100 liter batches using a mouse monoclonal antibody to the HIV-2 gp41 protein, which is a part of the gp160 protein. This purified HIV-2 gp160 reacts with high specificity and sensitivity with the anti viral antibodies of HIV-2 infected humans by several criteria such as i) ELISA, ii) western blot analysis, and iii) radioimmuno-precipitation of metabolically labeled gp160 from the same cell line.