This paper deals with an agar-gel adaptation of the Perret-Novick iodometric titration of degradation products from beta-lactam antibiotics when they are submitted to a beta-lactamase action. Petri dishes containing an agar-gel make up from the classical iodine-iodide-starch system, initially present a dark blue colour. When beta-lactam compounds are opened by beta-lactamases from crude extracts or whole bacteria, the gel is locally destained. Two experimental variants of this procedure are proposed. This semi-quantitative, inexpensive, accurate and simple method is very convenient for routine experiments. Thus with crude extracts or even whole cells, information can be obtained about beta-lactamase specific activities, substrate profils or activity of inhibitors such as cloxacillin or clavulanic acid.