The inhibitory activity of the penicillanic acid sulfone Ro 48-1220 against group 1, 2b, and 2be beta-lactamases was evaluated. Ro 48-1220 inhibited TEM and SHV as effectively as clavulanate and tazobactam. It also inhibited group 1 beta-lactamases at lower concentrations than tazobactam. Ro 48-1220, at a concentration of 4 micrograms/ml, protected ceftriaxone and ceftazidime against strains producing group 1 and 2be beta-lactamases.