This practice determines potentially effective microbicides for use in cooling water systems using cooling water and deposits/biofilm obtained from the field. The addition of deposits/biofilms addresses the need to include the major source of microorganisms in cooling water systems. Even with this addition, laboratory results may not be totally predictive of microbicidal effectiveness in the field. This is because conditions in the field affecting microbicide effectiveness are difficult to mimic in the laboratory. These conditions that affect microbicide efficacy include blow-down rate, addition of makeup water, water hardness, hydrocarbon leaks, pH, sediment loading, dissolved solids, microbes in slime (biofilms), and deposits (salts, iron minerals, organics, and so forth) on surfaces. An additional factor is the difficulty in enumerating all microbes in the water due to the lack of adequate recovery media. Guidelines that address formation of and testing for surface-attached microbes (biofilms) may be found in Guide E1427, while a guideline for unconventional measurement of microbes is found in Guide E1326.