A galactan sulfate (GS) was isolated from an aqueous extract of the red seaweed Aghardhiella tenera and partially purified. GS inhibited the cytopathic effect of human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) in MT-4 cells at concentrations 10-fold higher than those required for the inhibition by dextran sulfate (MW 5000) of the cytopathic effect of HIV-1 and HIV-2 (50% inhibitory concentrations: 0.5 and 0.05 mu gml(-1), respectively). GS suppressed syncytium formation between MOLT-4 cells and persistently HIV-1- or HIV-2-infected HUT-78 cells at concentrations higher than 5 mu gml(-1). Like dextran sulfate (DS) and aurintricarboxylic acid (ATA), GS inhibited the binding of HIV-I to the cells and the binding of anti-gp120 mAb to HIV-1 gp120. Like DS and ATA, GS proved active not only against HIV-I and HIV-2 but also against other enveloped viruses, i.e. herpes-, toga-, arena-, myxo- and rhabdoviruses. GS represents a natural polysaccharide with broad-spectrum activity against a number of important viral pathogens.