A set of dipeptides containing the amino acid residues beta-chloroalanine and propargylglycine, which are mechanism-based inactivators of purified microbial enzymes (alanine racemase and cystathionine gamma-synthase, respectively), have been synthesized, and their antibacterial properties in vitro have been evaluated. Dipeptides containing a single beta-chloro-L-alanyl residue (e.g., 3, 5, 9, and 10) or a single L-propargylglycyl residue (e.g., 12 and 15) are potent antibacterials. The in vitro antibiotic activity of beta-chloro-L-alanine and of L-propargylglycine is increased as much as 4000-fold by incorporation of these residues into a dipeptide. Compounds that contain only a single enzyme-inactivating amino acid together with a second L-alanyl residue (3, 5, 12, and 15) have a restricted range of activity: of the species tested, only Streptococcus agalactiae, Staphylococcus aureus, and Staphylococcus epidermidis are sensitive. However, peptides that contain two suicide-substrate residues [e.g., beta-Cl-LAla-beta-Cl-LAla (8) or LppGly-LppGly (18)] are broad-spectrum antibacterials; as many as 12 different species of the 16 surveyed are sensitive. Dipeptides that contain an amino-terminal L-methionyl (9) or an L-norvalyl (10) residue and a carboxy-terminal beta-chloro-L-alanyl unit are also effective against a large number of organisms; the spectra of activity are like those seen for 8 and 18. A "mixed" dipeptide [beta-Cl-LAla-LppGly, (21)] gives apparent synergism of antibiotic action of beta-chloro-L-alanine and of L-propargylglycine when these two residues are incorporated into a single structure. Peptides of the D,D configuration (4, 6, 13, 16, and 20) and ones of L,D stereochemistry (e.g., 7) are not antibacterials. Peptides containing one (11 and 14) and two (17) D,L-propargylglycyl residues are unresolved sets of diastereomers; the mixtures of compounds are between two- and fourfold less active than the correspondingly resolved L,L dipeptides (12, 15, and 18). These findings are consistent with a mechanism of action for these antibiotics involving stereoselective processing of the peptidyl unit in vivo.