Two broad roles have been revealed for the CD4 molecule. It serves as a receptor for both class II major histocompatibility complex molecules and human immunodeficiency virus (HIV). Upon binding class II major histocompatibility molecules, CD4 functions to enhance T-cell activation. By binding to CD4, HIV gains entry into the cell. We have used a chimeric molecule of CD4 and lymphocyte function-associated antigen 3 (LFA-3), CD4PI, which lacks a membrane-spanning domain and is instead anchored in the membrane by linkage to glycosyl-phosphatidylinositol. To further define the structural attributes of viral receptors, and specifically those of CD4 required for HIV infection, we have expressed CD4PI and CD4 in a human T-cell line HSB-2. We find that CD4PI is able to mediate infection of these cells by HIV with similar, if not greater efficiency, compared with wild-type CD4. Thus the membrane-spanning region of CD4 is not required for HIV infection, and a lipid-anchored protein can serve as a viral receptor.