A series of five laboratory performance studies on the analysis of chlorobiphenyls (CBs) and organochlorine pesticides (OCPs) has been conducted within the three year EU-QUASIMEME (Quality Assurance of Information for Marine Environmental Monitoring in Europe) programme. The results of the Rounds 4 and 5 and the overall results of this programme are discussed, It was shown that laboratories with a positive attitude towards qualtiy control could improve the accuracy of their data substantially. This is mainly explained by the learning element of the programme, which consisted of giving detailed advice and guidance to the participants and the organisation of a workshop. Two distinct recommendations are given to managers of marine monitoring programmes. To obtain a higher accuracy in monitor programmes, particularly for the component dealing with temporal trend studies, it is recommended that a small group of qualified laboratories should be selected to carry out such a task. Secondly the currently available analytical methods for CBs and OCPs do not allow the production of very accurate results when concentration are below 1 mu g/kg. (C) 1997 Science Ltd. All rights reserved.