Environmental Committee of the Helsinki Commission recommended that all laboratories reporting data to the Baltic Monitoring Programme should establish the inhouse quality assurance systems to ensure the acquisition of reliable and relevant data. The paper presents some aspects of quality assurance system as operated in the Marine Laboratory of the Institute of Meteorology and Water Management. An attempt to produce laboratory reference material of natural seawater sample was made and here the results of analyses are presented. Routine control charts (x-charts) are discussed with specjal reference to scientific crew stability and performance during monitoring cruises. Participation in intercalibration exercises is mandatory for HELCOM laboratories. The results of the recent intercomparison exercise of nutrient analyses within QUASIMEME programme are presented in this paper, too.