MATERIALS AND METHODS

Ferrichrome and ferrichrome A were isolated from cultures of Ustilago sphaerogena and the deferri deny atives prepared as previously described (8). Ferniox amine B was prepared from defernioxamine mesylate. N, N', N―-triacetylfusarinine C (tniacetylfusigen) was obtained from cultures of Pencillium sp.. and its iron chelate prepared as previously described (10). All sid erophore solutions were assayed spectrophotometnically using the following millimolar extinction coefficients: ferrichrome, 2.9 at 425 nm; ferrichrome A, 3.4 at 440 nm; triacetylfusarinine C, 3.0 at 440 nm; fenrioxamine B, 2.5 at 430 nm. Defernisiderophore solutions were as sayed by the addition of 0.1 ml of a solution of 0.01 M ferric perchlorate in 0.1 N nitrilotniacetic acid to 2.9 ml of the deferrisiderophore solution, and reading the ab sorbance at the appropriate wavelength (above) against