In vitro and in vivo studies have shown that phthalocyanine tetrasulfonate (PcTS), a cyclic tetrapyrrole compound, is an efficient antiscrapie drug. To investigate the spectrum of PcTS against prion diseases, we tested the effect of PcTS on two mouse-adapted human strains. We also tested PcTS in rodents infected with two scrapie strains (139A and 263K). PcTS treatment significantly prolonged mean survival times of all infected animals. These results show that PcTS is effective on different prion strains, confirming its potential use for prion therapy.