Since 2018, studies of marine litter and microplastics in the Gulf of Finland of the Baltic Sea have been carried out by scientists from Russia, Finland and Estonia as part of the Trilateral Gulf of Finland Scientific Cooperation. In December 2021, work was completed on the intercalibration of sampling methods and laboratory analysis of microplastics in water and bottom sediments of the Gulf of Finland by scientists from laboratories of the three countries. Interesting results have been obtained, which will be published in the near future. The water area of the Gulf of Finland is characterized by a high amount of organic matter, because The Baltic Sea is a eutrophic water body. The study has shown that polymer particle identification equipment and the use of the correct chemicals to remove organic matter while not damaging the plastic itself are crucial in the laboratory sample processing procedure. Errors in the use of reagents of insufficient density, as well as chemical glassware unsuitable for this type of research, lead to a strong overestimation / underestimation of the results of concentrations of microplastic particles. Also, a high importance of the number of laboratory processing steps of samples and their analysis is shown for the measurement accuracy: the greater is the number of processing steps, the greater is the measurement error. The work on unification and standardization of methods for monitoring microplastics within the framework of the Expert Group on Marine Litter from Russia in HELCOM (Helsinki Commission for the Protection of the Baltic Sea Environment) is carried out by the scientific group of the Laboratory for Plastic Pollution Research (PlasticLab) at the Russian State Hydrometeorological University lead by Associate Professor Alexandra Ershova.