From our investigations the following compounds have emerged as particularly potent and selective inhibitors of HIV replication: sulphated polysaccharides (i.e. heparin, dextran sulphate, pentosan polysulphate), dideoxynucleoside analogues such as the 3′-azido- and 3′-fluoro-substituted 2′,3′-dideoxyribosides of both purines (i.e. guanine, 2,6-diaminopurine) and pyrimidines (i.e. uracil, thymine), and the 9-(2-phosphonylmethoxyethyl) derivatives of adeninc, 2-monoaminopurine and 2,6-diaminopurine. All these compounds yield great promise for the treatment of retro virus infections in humans. Whereas the sulphated polysaccharides interfere with the virus adsorption process, the nucleoside analogues (following intracellular phosphorylation to their 5′-triphosphate) appear to be targeted at the reverse transcriptase.