Starting from November 1990, an international External Quality Assessment Scheme (EQAS) for immmunoassays of tumor markers has been organized. Presently, 238 laboratories from France, Germany, Italy, Japan and Spain participate in the scheme. In this report the main features of the EQAS and data processing are outlined. Results collected during the 1992-cycle allow evaluation of the state of the art of AFP, CEA, CA 19-9, CA 15-3, GA 125 and PSA immunoassays. According to their analytical performances the 6 tumor marker immunoassays can be classified into several groups, the first including AFP and CA 15-3 for wich both total variability and within-kit agreement are good. For CEA assay, performance can be considered as satisfactory even though further improvements of between-lab agreement would be welcome. For the 3 other tumor markers, the higher total variability indicates an urgent need for a better standardization by improvement of either both within-kit and between-kit agreements (CA 19-9) or between-kit agreement mainly (PSA, CA 125).
