The FDA has recommended that all blood collected in the USA should be screened for antibody to hepatitis B core antigen (anti-HBc) and for raised alanine aminotransferase (ALT) as possible indicators of non-A, non-B hepatitis carriage. As part of an assessment of the medical and economic implications of such a screening programme, we have screened 1,742 regular blood donors for ALT and 2,086 (including the same 1,742) for anti-HBc. 42 (2.4%) of the 1,742 donors had ALT levels above 45 units/l. Clinical assessment of 33 of these revealed that 26 exceeded their ideal body weight by more than 10% and 15 by more than 20%. 11 admitted to an alcohol intake of over 40 g daily. In all, 82% of donors with raised ALT had a ‘non-viral’ clinical explanation for this abnormality. Anti-HBc was detected in 42 (2.0%) of the 2,086 donors screened. 27 (64%) also had anti-HBs, and 11 (26%) had anti-HBe. There was no overlap between donors with raised ALT and those with anti-HBc. Combined screening would lead to a loss of at least 4.4% of donations in the population studied. In view of the medical and economic implications of the introduction of these screening tests, and the poverty of data on the clinical significance of post-transfusion non-A, non-B hepatitis, we conclude that such a screening programme cannot be justified at present. Further studies are required, including a prospective controlled trial of the effects of screening.