In the presence of a reporter substrate, the progressive inactivation of an enzyme was easily studied by directly transmitting absorbance readings to a microcomputer. Pseudo-first order rate constants as high as 0.3 sec-1 were rapidly and accurately measured. When utilization of the reporter substrate did not exceed 10%, the rate of the reaction (vt) could be considered as proportional to the active enzyme concentration at any time during the analysis and the decrease of vt was first order with time. This simple method was used to follow the inactivation of beta-lactamases (EC 184.108.40.206) by various physical and chemical agents. When a large proportion (30-80%) of reporter substrate was destroyed, a correction was introduced to account for the corresponding decrease of its rate of utilization. This enabled experiments to be performed with a DD-peptidase and a substrate exhibiting a low delta epsilon upon hydrolysis. For the first time, the inactivation of a penicillin-sensitive enzyme by a beta-lactam could be continuously and directly observed. Finally, the method was extended to the study of hysteresis phenomena.