The new Micro Particle Enzyme Immunoassay technique (MEIA, IMx HBc, Abbott) has been recently introduced for the detection of the antibody to hepatitis B core antigen (anti-HBc). To evaluate the feasibility of using the IMx-HBc, we carried out comparison tests with this method and RIA using various sera. Results obtained were as follows: In the test of 813 sera, 427 (52.3%) were positive by both IMx and RIA, two (0.2%) were positive by RIA only, and the remaining 384 (47.3%) were negative by both methods. A good correlation between IMx and RIA was observed with a coincidence rate of 99.8% and a correlation coefficient of 0.969. Examination of diluted sera using IMx revealed that IMx can be used as efficiently was RIA: high anti-HBc titer was found among HBsAg carriers, but not among non HBsAg carriers. Anti-HBc was assayed within 45 minutes by IMx, and the procedure was simple because of the auto analyser used in this method. These results indicate that the sensitivity of IMx is equivalent to that of RIA, and that it is easier to use than RIA.