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Notes: Summary.  Second generation enzyme immunoassays (EIA-2) for antibodies to hepatitis C virus (anti-HCV) have a higher specificity and sensitivity than first generation enzyme immunoassays (EIA-1). We studied how many anti-HCV-positive blood donors were missed by the EIA-1, how many were false positive, how false-positive donors should be dealt with and how the results of the EIA-2 correlate to demographic data and serum alanine aminotransferase (ALT) level. A total of 208, 544 northern German blood donors, not preselected for anti-HCV negativity, were tested for anti-HCV with EIA-2 and, if repeatably reactive (rr), were retested with a licensed supplementary test (RIBA-2). 0.43% of the donors were EIA-2 rr, but only 0.12% of women and 0.09% of men were RIBA-2 positive. RIBA-2 positivity rates were very low in donors 18 to 27 years old (0.03% and 0.05%) and rose with age in women but not in men. Infected women were significantly more often Rhesus-negative than men. The rate of unspecifically positive EIA-2 results (entirely negative in RIBA-2) increased with age in both sexes and did not correlate with ALT. The ALT distribution was age-dependent with a different pattern for men and women. Confirmation of EIA-2 results with RIBA was rare when ALT was low and frequent when ALT was high. ALT screening before introduction of Anti-HCV detected one out of six infected donors. To exclude this one infectious donation, 46 uninfected donations had to be excluded in addition. Only 8% of the then RIBA-2-positive donors were not detected by EIA-1. Apparent seroconversions in EIA-2 are usually not specific: only 1 out of 66 apparent seroconversions could be confirmed by RIBA-2 suggesting recent HCV infection. 0.15% of the donor population showed an inconsistent EIA-2 pattern during follow-up. We conclude that donors should not be excluded from further donations, even on the basis of multiple EIA-1 positive results or on the basis of only one EIA-2 positive donation. Anti-D-immunoglobulin prophylaxis may have been a source of infection in some Rhesus-negative women. ALT screening should not be discontinued because recent HCV infection can be detected earlier by ALT than by anti-HCV, but exclusion limits should be elevated to increase specificity and limit unnecessary exclusion of donations.