In vitro $eta$-lactamase inhibitory activity of 6-exomethylenepenam compounds ( 1, 2, 3, 4 and 5) was compared with clavulanic acid, sulbactam and tazobactam. The inhibitory activity of compound 3 was stronger than those of sulbactam and clavulanic acid against Type I and II enzymes and stronger than tazobactam against Type III, IV, TEM enzymes. The inhibitory activity of 5 was stronger than sulbactam and clavulanic acid against Type I and II enzymes and stronger than tazobactam against Type III, and IV enzymes. The in vitro antimicrobial activity of 3, 4 and 5 combined with ampicillin and cefoperazone was compared with the sulbactam against $eta$-lactamase producing 27 strains. But, synergistic activity of 3 and 5 was inferior to tazobactam.