We compared four preparations of recombinant HIV-1 envelope glycoprotein: mammalian (Chinese hamster ovary cells) gp120 (Celltech); baculovirus gp120 from American Biotechnologies Inc. (ABT) and from MicroGeneSys (MGS); and baculovirus gp160 (Institute of Virology, Oxford, UK). Each envelope glycoprotein binds to a neutralizing monoclonal antibody (MAb) directed against the V3 loop, confirming the integrity of this type-specific neutralization epitope. MGS gp120 binds abnormally well to a MAb which recognizes an epitope preferentially exposed on denatured gp120. Consistent with this finding, MGS gp120 binds to soluble CD4 (sCD4) with an affinity 50–100-fold lower than that of Celltech gp120. The affinity of Celltech gp120 from sCD4 is 2.3 nM, indistinguishable from that of gp120 extracted from HIV-1 virions. Baculovirus gp120 (ABT) and gp160 also have a high affinity for sCD4. A significant proportion of anti-gp120 antibodies in HIV-positive human sera recognize epitopes that are dependent on the mammalian glycosylation pattern, and a human HIV-positive serum inhibits the binding of mammalian gp120 to sCD4 five- to 10-fold more potently than it inhibits baculovirus gp120 binding to sCD4.