ABSTRACT.    Protein disulfide isomerases (E.C. 5.3.4.1) are multidomain multifunctional enzymes belonging to the thioredoxin superfamily conserved through evolution, typically found, in eukaryotes, in the endoplasmic reticulum, favouring folding of nascent proteins by catalyzing oxidation of thiol groups and isomerization of disulfide bonds. In some cell types, PDI may also display chaperone activities and locate in the cytoplasm or at the cell surface. The Plasmodium falciparum PDI was initially isolated in the laboratory (Pf52) by affinity chromatography over the antiplasmodial compound DS-61, a 1,4-bis(3-aminopropyl)piperazine derivative displaying an IC₅₀ of ˜100nM on P. falciparum growth (Florent et al., 2000, FEBSLetters 484, 246). Pf52-PfPDI displayed the typical PDI structure in five domains a-b-b'-a'-c, in which a and a' correspond to the two thioredoxin active site domains. Pf52-PfPDI was expressed throughout the parasite erythrocytic and was mainly localized into the endoplasmic reticulum. Recombinant Pf52-PfPDI displayed oxidase/isomerase, reductase and chaperone activities, with comparable parameters as those found for either purified native bovine PDI or human recombinant PDI. DS61 inhibited PfPDI oxidase/isomerase activity (IC₅₀ of 430μM) but not that of human recombinant PDI. DS61 could not be tested on the other PfPDI activities. The discrepancy between this value and the in vitro anti-plasmodial activity of DS61 (IC₅₀ of 100nM) suggests that the oxidase/isomerase activity of PfPDI may not be the only target of DS61 in P. falciparum. Other PfPDI activities - not yet tested - or other Plasmodium proteins could also be targeted by DS61.