The kinetic parameters for the maltose transport system in Escherichia coli K12 were determined with maltose and maltotriose as substrates. The system exhibits an apparent Km of 1 muM for maltose and 2 muM for maltotriose. The V of entry was determined as 2.0 and 1.1 nmol substrate/min per 10(8) cells. Mutations in lamB, the structural gene for the receptor protein of phage lambda, increased the Km for maltose transport by a factor of 100-500 without influencing the maximal rate of transport. Maltotriose is no longer transported in these lamB mutants. The maltose-binding protein, an essential component of the maltose transport system, was found to exhibit substrate-dependent fluorescence quenching. This phenomenon was used to determine dissociation constants and to estimate the rate of ligand dissociation. A Kd of 1 muM for maltose and of 0.16 muM for maltotroise was found. From the comparison of the kinetic parameters of transport of maltose and maltotriose in wild-type and lambda-resistant mutants with the binding constants for both sugars to purified maltose-binding protein, we conclude that the lambda receptor facilitates the diffusion of maltose and maltodextrins through the outer membrane.