Other approaches in addition to existing treatment of HIV infection need to be evaluated. One approach may be to block HIV infection by interrupting the assembly of the virus within the cell or the budding of virus from the membrane of the infected cell. In addition, blocking the attachment of HIV to its cellular receptor may offer another point of attack. HIV binds to the CD4 receptor on the target T4 lymphocyte and the envelope glycoprotein of the virus (gp120) is capable of high affinity binding to CD4. Any agent that prevents the attachment of gp120 to the CD4 receptor should be able to block virus transmission and spread. Recently, scientists have succeeded in producing highly purified recombinant soluble human CD4. Recombinant CD4 is capable of binding to HIV envelope protein (gp120) and inhibiting HIV infectivity in test tube studies. Potential therapeutic benefit in patients with HIV infection could be derived from either or both of these biologic effects. In order to extend the length of time that rCD4 stays in the body, the compound has been modified by combining it with a human immunoglobulin of the IgG1 class (IgG).