A retrospective study was undertaken to investigate the infectivity of blood donations that were HCV RIBA indeterminate or reactive for 'anti-HBc only'. Samples from 6032 blood donations were tested for anti-HBc; 17 of them were positive for anti-HBc but were negative when tested for anti-HBs. None of the recipients of the red cells from these donations, when tested 9 months after transfusion, had evidence of HBV infection. Samples from 3000 blood donations were tested for HCV antibodies using RIBA-3; 53 had single lines. Samples taken 9 months after transfusion from the recipients of the red cells from these donations were tested for evidence of infection with hepatitis C, and all were negative.