This report is concerned with preservation of the microbiological quality of the finished water following implementation of treatment techniques changes for reduction of trihalomethanes(THMs). An initial survey of utilities serving more than 75,000 individuals was conducted in early 1981 to determine which utilities were experiencing high THM concentrations, what control strategies were in reducing THM concentrations and maintaining microbiological quality in finished water. A field monitoring program aimed at examination of microbiological characteristics before, during and after changes were made for THM control, was conducted at three large utilities. Testing conducted by the utilities included temperature, pH, turbidity, and disinfectant residual. Testing results and conclusions derived from the results are summarized in this report, with detailed results available in three separate reports prepared for each field monitoring site. A Guidance Manual intended for use by utilities planning to make treatment technique changes for THM control is also included.