A method is described which may be used to compare the relative activity of β-lactam antibiotics against gram-negative bacilli. The method offers several important advantages over conventional methods of evaluation of the antibacterial activity of β-lactam agents: the bacterial inoculum is simply and accurately standardized; the response of the culture is continuously monitored and information is obtained on both the intrinsic activity of the agent and its susceptibility to entero-bacterialβ-lactamases. In this way, differences in the antibacterial activity of compounds which appear similar on the basis of more conventional tests may be reveal