Rhizoferrin-mediated iron uptake was studied in two different classes of organisms: a rhizoferrin producing fungus, Absidia spinosa (Zygomycetes), and a ferric rhizoferrin utilizing bacterium, Morganella morganii (Enterobacteriaceae). The uptake of iron rhizoferrin and some of its metal analogs (chromium, rhodium, gallium), was followed and kinetic parameters measured in A. spinosa. These metal ion complexes were taken up in a concentration- and energy-dependent manner indicative of an active transport system. The uptake of the kinetically inert chromium and rhodium and reductively inert gallium complexes suggests a variation of the so called ‘shuttle’ mechanism may be operative. The recognition of one geometrical isomer of chromium-rhizoferrin but not another argues for a degree of stereospecificity in the uptake process. A growth promotion plate assay was used to examine metal-rhizoferrin uptake in M. morganii. The results indicate that a number of factors including the nature of the chelating agent (e.g. bipyridyl or EDDHA) used to induce iron deficiency need to be considered before these simple plate assays can be reliably used to indicate the presence or absence of a particular siderophore uptake system.