The ferrichrome-iron receptor encoded by the fhuA gene of Escherichia coli K-12 is a multifunctional outer membrane receptor required for the binding and uptake of ferrichrome and bacteriophages T5, T1, phi 80, and UC-1 as well as colicin M. To identify domains of the protein which are important for FhuA activities, a library of 31 overlapping deletion mutants in the fhuA gene was generated. Export of FhuA deletion proteins to the outer membrane and receptor functions of the deletion proteins were analyzed. All but three of the deletion mutant FhuA proteins cofractionated with the outer membrane; no FhuA proteins were detected in outer membrane preparations or in cell extracts when the deletions spanned amino acids 418 to 440. Most deletion proteins were susceptible to cleavage by endogenous proteolytic activity; some degradation products were detected on Coomassie blue-stained gels and on Western blots (immunoblots). Receptor functions were measured with the mutated genes present on multicopy plasmids. Two deletion mutants, FhuA delta 060-069 and FhuA delta 129-168, conferred wild-type phenotypes: they demonstrated growth promotion by ferrichrome and the same efficiency of plating of bacteriophages as that of wild-type FhuA; killing by colicin M was also unaffected. For FhuA delta 021-128 and FhuA delta 406-417, reduced sensitivity to colicin M was detected; wild-type phenotypes were observed for all other FhuA functions. Deletions from amino acids 169 to 195 slightly reduced sensitivities to bacteriophages and to colicin M; ferrichrome growth promotion was unaffected. When deletions extended into the region of amino acids 196 to 405, all FhuA functions were either reduced or abolished. The results indicate that selected regions of the FhuA protein have receptor activities and demonstrate the presence of both shared and unique ligand-responsive domains.