Bacillus megaterium strain ATCC 19213 secretes a ferric-chelating secondary hydroxamic acid, whereas a mutant (strain SK11) derived from it cannot produce a hydroxamate. Strain SK11 could be cultivated in a sucrose-mineral salts medium (treated with Chelex 100 to reduce trace metals) in the absence of added hydroxamate, if the inoculum was high. The lowest iron supplements necessary for maximal growth of both strains were equivalent (0.01 to 0.04 mug of iron per ml). Addition of either aluminum (0.5 mug/ml) or chromium (0.1 mug/ml) to the medium prevented full growth of strain SK11 at the minimal iron concentration, although elevated iron (1 mug/ml) reversed this inhibition. The iron-free secondary hydroxamate, Desferal, also abolished aluminum and chromium inhibition of strain SK11, producing maximal population densities at the low iron concentration. Growth of the hydroxamate-producing strain 19213 was not altered significantly by the aluminum or chromium levels which inhibited strain SK11. However, strain 19213 responded to these metals by increasing its secretion of a secondary hydroxamate. It was concluded that aluminum and chromium interfered with iron incorporation, either directly or by formation of nonutilizable aggregates with iron. The secondary hydroxamates may have overcome this interference by solubilization of iron for delivery to a single uptake process, or the ferric-hydroxamate chelate may enter the cell by an alternate route.