A fully automated microparticle enzyme immunoassay, IMx HAVAB, was developed for the detection of antibody against hepatitis A virus (anti-HAV). In the IMx HAVAB assay which is run on the IMx instrument, 24 tests are completed in less than 45 minutes. IMx HAVAB sensitivity was 18-25 World Health Organization U/l and was more sensitive than the commercial RIA or EIA, HAVAB and HAVAB EIA, respectively. Specimens from blood donors, diagnostic and hospital patients and individuals with a variety of infectious and immune diseases were tested in parallel with IMx HAVAB and RIA or EIA. Overall agreement of 99.9% (2118/2121) was obtained. Prevalence of anti-HAV tested by IMx ranged from 12.3% in volunteer blood donors in St. Louis to 64.3% for hospital patients in New York City. Discordant specimens were reactive by IMx HAVAB but borderline negative by EIA or RIA, due to the better sensitivity of the IMx assay. IMx HAVAB detected both IgM and IgG subclasses of anti-HAV. Serial bleeds from six intravenous drug users with acute HAV infection were tested over 8 months for the presence of anti-HAV. At all time points, patients were strongly reactive for anti-HAV (titers greater than 1/1000). Anti-HAV titers rose during the first 20 weeks after presentation of symptoms and then declined with time.