Serum alanine aminotransferase (ALT) activity and antibody to hepatitis B core antigen (anti-HBc) were proposed as surrogate markers of non-A, non-B (NANB) infection. In this study we analyzed 649 consecutive repeat blood donors to define the possible exclusion rate if both surrogate markers were implemented in our Blood Service, and to assess risk factors associated with elevated ALT levels. One hundred and seven blood donors (16.5%) had slightly elevated ALT levels (higher than the upper reference value, but less than twice this level), but only 15 (2.3%) had a level higher than mean log + 2.25 SD. Seventy-seven (11.8%) resulted anti-HBc positive. Blood donors with elevated ALT levels and those who were anti-HBc positive belonged to different populations, being only 6 (0.9%) positive for both surrogate markers. Only two known donors (0.3%) resulted anti-HCV positive, and each of them was implicated in one of the four post-transfusion hepatitis (PTH) cases observed in 200 recipients of blood from these 649 donors. Both were negative for anti-HBc but one had elevated ALT levels. Male sex, age, alcohol use and obesity resulted all independently and significantly associated with elevated ALT levels. For both alcohol use and body weight we observed a significant linear relationship with serum ALT levels. These findings suggest that in our Region the exclusion of blood donors with ALT levels above the reference value, or those anti-HBc positive, would exclude an unacceptably high rate of blood donors without proven evidence of post-transfusion hepatitis prevention.(ABSTRACT TRUNCATED AT 250 WORDS)