Heparin\heparan sulfate and heparin-mimetic sulfated oligosaccharides (S- oligoS) were studied in vitro to elucidate the structural basis of their specific modulations of protein and cell membrane functions, using physi- cal, biochemical, and biological methods. In one such study, strategy was based on our view that heparin-like molecules play a role in the molecular mechanisms underlying the infectivity of human immunodeficiency virus (HIV-1): If the known inhibition of the cytotoxic (CT) and syncy- tium-formimng (SF) infectiousness of HIV-1 by S-oligoS were governed by a structural specificity, then rationale for clinical usefulness of a putative S-oligoS agent against AIDS would be enhanced. Here first, structural specificity in the inhibition of HIV-1 by S-oligoS was demonstrated, using an HIV-inhibitory anticoagulant pharmaceu-tical which comprised a mixture of S-oligoS components(Cps) from sulfated xylan. Highly active (HA) Cp were then purified and this specificity was separable from anticoagulantly active molecules. Now 1) The minimum-sized HA-Cp, CpF, (EC50 vs SF and CT= 100 and 200 ng/ml) was purified. Molecular mass of Cps was determined by S-E ultra- centrifugation of monodansyl derivatives. Minimum mass of anti-HIV-1 S- oligoS was approximately 4500 and a smaller S-oligoS, HA vs CT but not SF, had mass of nonomer. 2) Cp contain S-alpha 1,2 D-GlcA monomers on the S-xylose chain. Molecular modeling revealed that such GlcA could impart local structures resembling those in heparin. Analysis for GlcA by o-nitrophenylhydrazide reaction revealed unexpectedly that Cps had multiple GlcA, supporting a view that their heparin-mimetic action redounds from GlcA-containing structures. 3) Anti-HIV-1 activity appears to reside in a tetradecaglu-curonoxyloside containing 2 or 3 GlcA. 4) Methods upscaling preparation of CpF were developed and are being modified for clinical application.