[55Fe] ferric enterobactin to cells of Escherichia coli BN3040 NalR iuc. In contrast, serum had essentially no effect on the rate of these reactions mediated by aerobactin. Three purified serum proteins, human serum albumin, bovine serum albumin, and humanimmunoglobulin, were comparable to human serum in their selective ability to interfere with the transfer of 55Fe from [55Fe] ferric enterobactin to E. coli BN3040 NalR iuc. The inhibitory effect of human serum albumin on the enterobactin-mediated transfer of iron from [55Fe] transferrin was enhanced bypreincubation of the protein with the siderophore. Pretreatment of the bacterial cells with human serum