In recent years, a microcalorimetric test was developed for detg. the microbial activity of biofilm samples on metals. The major advantage of this test is that the biofilm remains on the surface or does not need further treatment for the measurement, thereby enabling real online measurements of microbial activity in the biofilm. This technique also showed a capability for testing biocide treatment and screening and optimizing countermeasures against biofouling and microbially influenced corrosion. The use of this test to det. the biol. and chem. activity in biofilm samples on mild and stainless steel elements of the water circulation systems is described, and the recommendations are given with respect to the evaluation of methods for reducing the microbial activity and viable cells in biofilms.