FepA, an 81 kD protein of gram-negative bacteria, transports iron across the outer membrane in a manner that requires a number of accessory proteins in the bacterial cell envelope. For this reason, it is desirable to study protein dynamics in the intact cell. Spin-label binding sites were introduced into the cell-surface region of FepA by site-directed mutagenesis. Reaction conditions are being sought that will allow selective labeling of these sites in line, intact E. coli.