Two novel fluorescence methods for the assay of peptide transport in microorganisms are described. These methods are compared with conventional radiotracer assays, and certain advantages of the fluorescence techniques are highlighted. Peptide uptake in the yeast Saccharomyces cerevisiae is investigated. It is shown that peptide and amino acid uptake are distinct, intact peptide accumulation is demonstrated, and the presence of a single main peptide permease, which handles a range of di- and tripeptides, is demonstrated. A mutant deficient in this permease is isolated on the basis of resistance to the dipeptide-mimetic antibiotic bacilysin. Peptide uptake in Streptococcus faecalis is also shown to be distinct from amino acid transport, and intact peptide accumulation is again demonstrated. A strain of Strep, faecalis (faecium) is shown to possess two peptide permeases, a high-rate system for di- and tripeptides, and a low-rate oligopeptide system. Other strains of Strep, faecalis are shown to differ in their peptide transport specificity. Mutant strains deficient in peptide permeases or intracellular peptiaases are isolated on the basis of resistance to peptide mimetic antibiotics. The coupling of metabolic energy to peptide transport is investigated in Sacc. cerevisiae and Strep, faecalis. In both cases a proton-motive-force is implicated. Some preliminary investigations of the energy consumption of peptide transport in Strep, faecalis, implying the same stoichiometry for tri-, tetra- and pentaalanine uptake, are reported.