Properties of the enzyme which hydrolyses enterochelin (a cyclic trimer of 2,3-dihydroxy-N-benzoyl-L-serine) to 2,3-dihydroxybenzoylserine have been investigated with a view to resolving discrepancies between earlier reports. Enterochelin esterase, previously reported to consists of two components (O'Brien, I.G., Cox, G.B. and Gibson, F. (1971) Biochim. Biophys. Acta 237, 537-549), has been shown to be fully active in the absence of the so-called A component. The hydrolase described previously (Bryce, G.F. and Brot, N. (1972) Biochemistry 11, 1708-1715) as being able to break down enterochelin but not its iron complex, ferric-enterochelin, appears to be identical with the B component of enterochelin esterase. The single component enterochelin esterase corresponding to what was previously described as component B, hydrolyses both enterochelin and ferric-enterochelin. Under the assay conditions used, enterochelin is hydrolysed 2.5 times faster than the complex. Enzymatic activity is inhibited by N-ethylmaleimide and is lost rapidly at 37 degrees C. Activity is stabilized in the presence of ferric-enterochelin, enterochelin, dithiothreitol or certain protein fractions.