We investigated a new p24 antigen detection system by the chemiluminescence-enzyme-immuno-assay (CLEIA) and compared it with the conventional antigen-kits (A; Abbott, B; Coulter). The detection limit of CLEIA, A, and B were 4.3 pg/ml, 11.3 pg/ml, and 5.6 pg/ml, respectively. When the patient's sera and panel sera were examined by the three method, compatible results with the detection limit were obtained. CLEIA can be completed within 35 min, and is performed by an automatic system, which can treat many samples at one time. Therefore, this system is more suitable for screening of donor blood because of the shortening of the window period.