The effects of various amino acids on vancomycin MICs obtained with resistant enterococci was investigated by using broth dilution testing. For both the type A (i.e., possessing transferable resistance to teicoplanin and vancomycin) and the type B (i.e., possessing teicoplanin susceptibility and nontransferable vancomycin resistance) resistant strains, vancomycin MICs in the presence of glycine were substantially lower than those in unsupplemented broth (range of MIC decrease, 8- to 128-fold). No such effect was seen with Enterococcus gallinarum AIB-39 or with the susceptible control Enterococcus faecalis ATCC 29212. Further testing of two type B strains (E. faecalis V583 and V583-2) showed that certain other amino acids (i.e., D-methionine, D-serine, D-alanine, and D-phenylalanine) had effects similar to that of glycine. Results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis with membrane preparations from these strains revealed the production of vancomycin-inducible proteins of sizes comparable to those describe for other enterococcal isolates that exhibit acquired vancomycin resistance. Even in the presence of 0.2 M glycine, the inducible proteins were produced. These results indicate that certain amino acids specifically interfere with the mechanism(s) of acquired vancomycin resistance in enterococci and that the nature of interference probably involves inhibition or circumvention of the inducible proteins' functions.