Iron reduction and uptake was studied in wild-type and haem-deficient strains of Saccharomyces cerevisiae. Haem-deficient strains lacked inducible ferri-reductase activity and were unable to take up iron from different ferric chelates such as Fe(III)-citrate or rhodoturulic acid. In contrast, ferrioxamine B was taken up actively by the mutants as well as by the wild-type strains. At a low extracellular concentration, uptake was insensitive to ferrozine and competitively inhibited by Ga(III)-desferrioxamine B. Extracellular reductive dissociation of the siderophore occurred at higher extracellular concentrations. Two mechanisms appear to contribute to the uptake of ferrioxamine B by S. cerevisiae: one with high affinity, by which the siderophore is internalized as such and another with lower affinity by which iron is dissociated from the ligand prior to uptake.