African trypanosomes live free in the bloodstream and central nervous system of mammalian hosts and also within the midgut of the tsetse fly vectors which transmit them. The parasite plasma membrane represents the interface between both hosts and parasite, and trypanosomes accumulate many essential metabolites via specific transport processes. L-Methionine uptake by procyclic and bloodstream forms of Trypanosoma brucei has been measured and shown to be mediated by a transporter presenting similar characteristics in both forms of the parasite. The carrier shows, in both forms, a relatively high affinity for methionine (Km ca. 30 microM). The effect of inhibitors of ion gradients across the membrane indicated that the uptake process is likely to be dependent upon a proton motive force. Various methionine analogues were tested against the transporter and these have demonstrated that the recognition depends on the motif common to all amino acids, and an electronegative group at the position of the sulphur atom separated from the alpha-carbon atom by a two carbon spacer.