A sulfone .BETA.-lactam compound which acts as a .BETA.-lactamase inhibitor.

CP-45,899 [3,3-dimethyl-7-oxo-4-thia-1-azabicyclo(3,2,0)heptane-2-carboxylic acid, 4,4-dioxide [2S-(2alpha,5alpha)]] has low intrinsic activity against most Gram-positive cocci, Enterobacteriaceae and Pseudomonas. It inhibits Neisseria at concentrations of 0.1 approximately 6.2 microgram/ml. The combination of CP-45,899 and ampicillin inhibited Staphylococcus aureus and Enterobacteriaceae resistant to ampicillin by virtue of beta-lactamase activity. Combination of CP-45,899 and cephalothin was synergistic less often, and CP-45,899 did not act synergistically with carbenicillin or ticarcillin against Pseudomonas resistant to these agents. CP-45,899 acted synergistically with ampicillin against Bacteroides. Synergy of CP-45,899 and ampicillin was demonstrated at varying concentrations suggesting that it may significantly enlarge the antibacterial activity of ampicillin against resistant bacteria.