We report here on the construction and use of a novel human immunodeficiency virus (HIV) type 1 reporter vector, HIV-AP, that encodes human placental alkaline phosphatase. Upon staining with chromogenic alkaline phosphatase substrates 24 to 36 h postinfection, cells infected with HIV-AP develop an intense purple color and can then be counted under a dissecting microscope. Alternatively, HIV-AP infectivity can be quantitated and infected cells can be sorted by a fluorescence-activated cell sorter after staining with a fluorescent alkaline phosphatase substrate. The assay is rapid and accurate, has very low background in a variety of cell lines and primary cells, and is not restricted to use in human cells. Infectious HIV-AP can be pseudotyped by various HIV or murine leukemia virus envelope glycoproteins. Using this virus, we have addressed the long-standing question of CD4- independent infection of cells by HIV. Our results confirm the presence on a human osteosarcoma cell line of an alternative receptor for HIV infection that functions with an efficiency approximately 1/20 that of CD4.