After interviewing natives and migrants from the Amazon region of Brazil about plants traditionally used for treatment of malaria fever and/or liver disorders, we selected and identified 41 different species, including the native Bidens (Asteraceae). We have undertaken an antimalarial study of Bidens pilosa and other species of Bidens from abroad. The crude ethanol extracts (whole plant, leaves and roots) and the chloroform and butanol fractions from B. pilosa at concentrations of 50 microg/ml caused up to 90% inhibition of Plasmodium falciparum growth in vitro. In vivo the fractions caused partial reduction of Plasmodium berghei parasitemia in mice. The ethanol extracts from nine different Bidens species collected outside Brazil were tested, and seven inhibited parasite growth in vitro by 65-91%. As B. pilosa appears to be a promising antimalarial agent, we further characterized the substances responsible for such activity. HPLC analysis using a photo diode-array detector showed phenyl acetylene and flavonoids in the ethanol extract from the leaves and roots. The chloroform fractions from the roots, which caused 86% inhibition of parasite growth in vitro, contained a major component identified as 1-phenyl-1,3-diyn-5-en-7-ol-acetate. The association of antimalarial activity and the presence of acetylene compounds is discussed. In summary, all species of Bidens which had aliphatic acetylenes (6-14 each) were also very active, whereas extracts of B. parriflora and of B. bitternata with none or the three acetylenes, respectively as reported in literature, were inactive or had a borderline activity in vitro.