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A periplasmic arginine transport system that is a member of the ATP -dependent transport superfamily was identified in Pasteurella haemolytica. The gene encoding the periplasmic binding protein (lapT) was cloned and the protein overexpressed in Escherichia coli. LapT was purified to homogeneity using a modified osmotic shock procedure and anion-exchange column chromatography. Filter-binding assays established that LapT is an L-arginine-binding protein. Various amino acids were tested for their ability to inhibit L-arginine binding to LapT. When present in 100-fold excess, only L-arginine, D-arginine and citrulline competed with L-arginine for binding. Arginine transport in P. haemolytica whole cells was competitively inhibited by the same amino acids, suggesting that the LapT permease specifically transports L-arginine. The dissociation constant for the L-arginine-LapT complex was 170 nM and the stoichiometry of binding was approximately 0.8 mol L-arginine (mol LapT)-1. A polyclonal antibody raised against the purified protein permitted detection of LapT in P. haemolytica periplasmic fractions.