Resistance to beta-lactam antibiotics mediated by metallo-beta-lactamases is an increasingly worrying clinical problem. Candidate inhibitors include mercaptocarboxylic acids, and we report studies of a simple such compound, thiomandelic acid. A series of 35 analogues were synthesised and examined as metallo-beta-lactamase inhibitors. The Ki values (B. cereus enzyme) are 0.09microM for R-thiomandelic acid and 1.28microM for the S-isomer. Structure-activity relationships show that the thiol is essential for activity and the carboxylate increases potency; the affinity is greatest when these groups are close together. Thioesters of thiomandelic acid are substrates for the enzyme, liberating thiomandelic acid, suggesting a starting-point for the design of 'pro-drugs'. Importantly, thiomandelic acid is a broad-spectrum inhibitor of metallo-beta-lactamases, with a sub-micromolar Ki for all nine enzymes tested, except the Aeromonas hydrophila enzyme; such a wide spectrum of activity is unprecedented. The binding of thiomandelic acid to the B. cereus enzyme was studied by NMR; the results are consistent with the idea that the inhibitor thiol binds to both zinc ions, while its carboxylate binds to Arg91. Amide chemical shift perturbations for residues 30-40 (the beta3-beta4 loop) suggest that this small inhibitor induces a movement of this loop of the kind seen for other larger inhibitors.