Tagonist `Compound A' [4-nitrobenzyl 1-(3(N-methylphenylsulfonamido)-3-phenylbutyl)piperidin-4-yl(vinyl)carbamate] [41,42] was kindly provided by Dr. Martin Springer at Merck Research Laboratories, Rahway, NJ. The anti-gp120 monoclonal antibody 48d, which recognizes an epitope that overlaps the coreceptor-binding site, was kindly provided by Dr. James Robinson (Tulane University Medical Center) [4