BACKGROUND: Since various hematology analyzers apply common or different principles in complete blood counting, difference between measured values could developed according to the control material used in external quality assessment. Diagnostic Hematology Subcommittee has been using formalin fixed blood and Liquichek(TM) Hematology-16 (Bio-Rad, USA) as control material of external quality assessment alternately but recently significant difference of test results was found in some analyzers. We intended to select adequate control material showing similar test results in most analyzers. METHODS: Using fresh whole blood, formalin fixed blood, Liquichek(TM) Hematology-16 and CBC-4K (R&D, USA), 5 parameters (WBC, RBC, Hb, Hct, platelet) were measured in 4 hematology analyzers; CELL-DYN sapphire (Abbott Diagnostics, USA), Coulter LH750 (Beckman Coulter, USA), ADVIA 2120 (Siemens Diagnostics, USA) and Sysmex XE-2100 (Sysmex Co., Japan). Linearity, within-run precision and between-day precision of 4 materials for each parameter were evaluated at different analyzers. RESULTS: The between-day precisions for WBC of formalin fixed blood showed very high CVs of 6.5~13.5% in all 4 hematology analyzers. The within-run and between-day precisions for WBC and platelet of Liquichek(TM) Hematology-16 showed high CVs of 9.3%, 16% and 19.8%, 18%, respectively in CELL-DYN analyzer. But, CBC-4K showed a good linearity (r2=0.9953~0.9993) and precision (within-run CVs, 0~1.5% and between-day CVs, 0~2.0%) in all analyzers. CONCLUSIONS: Fresh whole blood, formalin fixed blood and Liquichek(TM) Hematology-16 are not appropriate for standardization of external quality control materials because of some different test results among analyzers. We conclude that CBC-4K with good performance in all hematology analyzer is adequate as external quality control material.