False-positive enzyme immunoassay (EIA) tests in blood donors receiving influenza vaccine were first reported in 1991. We conducted follow-up testing for 6 months of those donors with multiply reactive, but unconfirmed EIA (at least 2 positives in anti-HCV-1.0, anti-HIV-1, and anti-HTLV-I assays) with a history of recent flu vaccine to determine the duration of false positivity. Of 133,000 donors tested, 16 met study criteria; all 16 were reactive for anti-HCV, 10 were reactive for anti-HIV-1, and 12 were reactive for anti-HTLV-I. Fifteen donors were available for follow-up testing (using the original screening and supplemental tests): 10 (67%) reverted to negative for the 3 tests and 5 remained false positive for various markers at last sampling (3-6 months after vaccination). The mean duration of false positivity for those reverting to negative EIA test status, was 4.2 months (range 2-7 months) indicating a transient phenomenon and supporting studies which suggest a role for IgM in the mechanism.