Suramin is a potent inhibitor of the reverse transcriptase (RNA-directed DNA polymerase) of retroviruses, including the HTLV-III/LAV (human T-cell lymphotropic virus type III/lymphadenopathy-associated virus) reverse transcriptase. Although suramin is far from specific as a reverse transcriptase inhibitor and known to interact with a multitude of proteins and enzymes, it is able to suppress the replication and cytopathic effect of HTLV-III/LAV at concentrations which are nontoxic for the host cells and readily attainable in humans. Consequently, suramin is also able to block HTLV-III/LAV replication in patients. The mechanism of action of suramin at the molecular biological level, its mode of transport and accumulation by the infected host cells, and the bases for its rather selective virustatic activity remain, to a large extent, to be elucidated.