Effective microbiological control can be achieved by monitoring microbial levels as well as actual micro biocide treatment levels. The keys to measuring the effectiveness of any biocide program are the ability to quickly and accurately measure the microbiological activity in the cooling system. Measuring the actual biocide level in the system over time can also enhance the control of the system. This provides important information regarding the fate of the material in the system as well as the suitability of the biocide feed protocol selected. This paper will illustrate how a simple, rapid bioassay and an on-line chemical analyzer can be used to control the effectiveness of biocides in open recirculating cooling systems permitting the optimization of the biocide program to achieve the most cost effective control.