Oligopeptides and dipeptides are transported into Saccharomyces cerevisiae by a carrier-mediated system. In the dark, leucyl-p-nitroanilide (Leu-p-NA) and leucyl-leucyl-4-azido-2-nitrophenylalanine [Leu-Leu-Phe-(4N3,2NO2)] are competitive inhibitors of peptide transport by S. cerevisiae cells. The photolysis of yeast cells in the presence of Leu-p-NA or Leu-Leu-Phe(4N3,2NO2) at 350 nm results in an irreversible inactivation of peptide transport. Protection against this inactivation is afforded by an excess of trimethionine, a transported peptide. Photolysis with Leu-p-NA or Leu-Leu-Phe(4N3,2NO2) does not affect amino acid or sugar transport, and cell viability is maintained throughout the irradiation procedure. A 5-min irradiation of S. cerevisiae with 2.4 microM Leu-p-NA or 15 microM Leu-Leu-Phe(4N3,2NO2) causes 50% inhibition of trimethionine uptake. p-Nitroaniline, a possible hydrolysis product generated from Leu-p-NA by cellular peptidase activity, has no effect on peptide transport. An exogenous energy source is not required for photoinactivation. The results suggest that a component(s) of the peptide transport system of S. cerevisiae is irreversibly modified by photolysis with Leu-p-NA or Leu-Leu-Phe-(4N3,2NO2) and provide the first example of the use of amino acid p-nitroanilides as photoaffinity labels.