Investigations of peptide transport in Salmonella typhimurium are presented. A strain designated proB25, a proline auxotroph, grew on a variety of di, tri-, and tetrapeptides containing proline. In contrast (Pro)6, peptides acylated on the NH2 terminus and Ala-Pro-D-Ala did not satisfy the nutritional requirement of proB25 for proline because they were not transported. A derivative of proB25, strain TN87, deficient in a proline aminopeptidase and an X-Pro dipeptidase, was able to utilize only four of 25 proline-containing peptides investigated. The inability of TN87 to grow on most of these peptides was due to the lack of the requisite peptidase activity. Evidence for a functional dipeptide transport system in this strain is indicated by growth on Pro-Leu and Pro-Ala, and by growth inhibition by certain X-Pro dipeptides. Leu-Pro, Val-Pro, Met-Pro, and Arg-Pro cause temporary growth inhibition of strain TN87 whereas Gly-Pro, Ala-Pro, and Pro-Pro have no effect on growth. Evidence for a functional oligopeptide transport system is indicated by growth on Pro-Val-Gly and Pro-Gly-Gly and by uptake of label from L-methionyl-L-methionyl-L-[14C]methionine and L-alanyl-L-prolyl-[14C]glycine. The presence of multiple oligopeptide transport systems for certain proline-containing peptides was demonstrated using triornithine-resistant mutants (oligopeptide permease deficient) and competition experiments. Finally L-Ala-L-Pro-[14C]Gly is shown to be transported intact into strain TN87.