In 1994, the Environmental Committee of the Helsinki Commission recommended that all laboratories providing data within the monitoring programmes of the Baltic Sea (HELCOM BMP) and HELCOM CMP (HELCOM COMBINE since 1996) should implement quality assurance system in monitoring activities. Quality assurance system includes validation of applied analytical methods, that is evaluation of measurement errors (measurement uncertainty) under current laboratory conditions. Some elements of quality assurance system implemented at the Marine Chemistry Laboratory of the Institute of Meteorology and Water Management in Gdynia are presented regarding method validation, i.e. experimental determination of precision, bias and detection limit of analytical methods in use and experimental determination of measurement uncertainty related to sampling for nutrient analyses. Results of internal quality control are also presented as well as results of international analytical proficiency tests in which the laboratory participates as a form of external quality control.