Mutants ton A and ton B of Escherichia coli K12, known to be resistant to bacteriophage phi80, were found to be insensitive as well to albomycin, an analogue of the specific siderochrome ferrichrome. Ferrichrome at micromolar concentrations strongly inhibited plaque production by phi80. Preincubation with ferrichrome did not inactivate the phage. At a concentration at which ferrichrome allowed 90% inhibition of plaque formation, the chromium analogue of ferrichrome showed no detectable activity. Similarly, ethylenediaminetetraacetic acid, ferrichrome A, and certain siderochromes structurally distinct from ferrichrome, such as ferrioxamine B, schizokinen, citrate, and enterobactin, did not show detectable inhibitory activity. However, rhodotorulic acid showed moderate activity. A host range mutant of phi80, phi80h, was also inhibited by ferrichrome, as was a hybrid of phage lambda possessing the host range of phi80. However, phage lambdacI- and a hybrid of phi80 possessing the host range of lambda were not affected by ferrichrome. Finally, ferrichrome and chromic deferriferrichrome were shown to inhibit adsorption of phi80 to sensitive cells, ferrichrome giving 50% inhibition of adsorption at a minimal concentration of 8 nM. It is suggested that a component of the ferrichrome uptake system may reside in the outer membrane of E. coli K12 and may also function as a component of the receptor site for bacteriophage phi80, and that ferrichrome inhibition of the phage represents a competition for this common site.