Structure-activity relationships in a series of (5R)-6-triazolylmethylene penems with potent beta-lactamase inhibitory activity are described. In most cases, their in vitro synergistic activity with amoxycillin is superior to that of clavulanic acid, sulbactam and tazobactam (YTR 830). Against an Escherichia coli TEM-1 infection in mice, the compounds showed a broad range of potencies; an optimum polarity was found, however, which gave maximum potency.