Drinking water disinfectants used in large water treatment plants and in associated water distribution systems are regulated by the Biocidal Products Regulation (EU) 528/2012. According to the regulation, biocidal products need to be sufficiently effective. The criteria for efficacy are laid down in Guidance on the Biocidal Products Regulation, Volume II Efficacy - Assessment and Evaluation (Parts B+C). Author’s opinion is that not all the requirements in the current guidance version 3.0 (April 2018) are suitable for drinking water disinfectants as they are used in drinking water plants in the Nordic countries. The document presents reasoning behind these views and suggests modifications for the next version of the guidance.