A class of synthetic lignins (dehydrogenation polymers of p-coumaric acid, ferulic acid, and caffeic acid) inhibited cytopathogenicity of HIV-1 and HIV-2 infection. The ratio of cytotoxic to anti-HIV (human immunodeficiency virus) doses depended strongly on conditions during polymer preparation. The activity increased when polymers were treated with reducing agent NaBH4, whereas it decreased when treated with oxidizing agent ceric ammonium nitrate. The polymers inhibited expression of HIV-specific antigen in the infected cells and also inhibited HIV-binding to the cells, but not completely, even at doses that almost completely inhibited the HIV-induced cytopathogenic effect. These results suggest that lignin structure, regardless of whether synthetic or natural, may inhibit HIV replication by an unidentified process, and thus prove to be a new class of anti-HIV agents possibly effective in the treatment of AIDS (acquired immunodeficiency syndrome).