Abstract: A device and process for determining the effect of an antimicrobial agent or other compound on a microorganism growing in continous broth culture. The microorganisms are grown in a vessel of fixed volume (18), which is supplied with a constant inflow of nutrient broth selected from one of two resevoirs. The culture is first stabilized by drawing broth from resevoir (10), which contains nutrient broth only, and then at a particular time the inflow is switched to resevoir (12), which contains nutrient broth and the compound to be tested. By measuring the density of the culture, in particular by counting the organisms as they leave the drain (20), the growth rate of the microorganisms can be calculated as a function of time or compound concentration.