Amylomaltase, the inducible 4-alpha-glucanotransferase of Escherichia coli strain ML, has been purified to homogeneity. Its specific activity with a commercial maltose substrate was 500 mkat/kg protein (30 mumol glucose formed min-1 mg protein-1). The purified enzyme, dependent on buffer concentration, exists in interconvertible low-molecular-weight (apparent molecular weight 71000) and high-molecular-weight (apparent molecular weight 370000) forms. The specificity of amylomaltase has been redefined. Hitherto, the enzyme was thought to be a glucosyltransferase, catalysing the transfer of single glucosyl units, and maltose has been regarded as its most important substrate. Amylomaltase is now shown to exhibit both glucosyl-transfer and 4-alpha-glucanosyl-transfer specificity. 4-alpha-Glucanosyl chains containing up to at least nine glucosyl units can be transferred. However, it is concluded that the transfer reaction by which amylomaltase action was originally expressed, does not take place, i.e., Maltose + maltose in equilibrium Maltotriose + glucose and that maltose has a restricted role as a substrate. This may be due to the inability of maltose to function as a donor substrate, serving only as an acceptor substrate. It is confirmed that when a maltodextrin serves as a donor, that portion of the molecule transfered by the enzyme is that containing the nonreducing-end-group. Enzyme action on chromatographically pure maltose is characterized by a lag phase in the time course of glucose release. The lag pahse is overcome by addition of 'priming' (catalytic) concentrations of maltotriose or higher maltodextrins. An autocatalytic reaction mechanism involving the generation of primer molecules is proposed to explain the action of the enzyme on maltose. The redefined action pattern of amylomaltase is consistent with the redefined role of the enzyme in the utilization of exogenous and endogenous 1,4-alpha-glucans by E. coli.