This is the report of fifth-year trial of external quality assessment for immunoassay in Korea. We attempted one trials of external quality assessments in 1999. A total of 9 specimens were distributed with a response of 258 laboratories (77.7%). We used home-made pooled sera as the control material for all the immunoassay items. The results are summarized as follows. 1. Laboratories participating in external quality control program of immunoassay increased in its number from 225 to 236 laboratories compared to previous year 1998. 2. Abbott AxSYM analyser is most widely used for immunoassay tests. Use of chemiluminescence analysers, such as ACS 180 of Chiron Corp., Elecsys of Roche Corp., are increasing rapidly in some of test items and are being used as replacement for EIA analysers. 3. Chemiluminescence analysers are most widely used in measuring thyroid hormones. 4. Among member laboratories using same type of analysers showed large coefficient of variations in some of test items. We recommend that laboratories should use dilutional method for high value samples, but some may failed resulting large coefficient variation in some tests items. 5. Member laboratories are using automated analysers such as nephelometer or turbidimeter instead of RID plates (manual methods) for fast and accurate result reports in analysis of serum immunoglobulin (IgG, A, M, C3, C4).