Blood samples were collected from 7572 healthy volunteer blood donors from 21 of the 27 Indonesian provinces, and tested for antibodies to hepatitis C virus (anti-HCV) using the new second-generation enzyme immunosorbent assay, and also tested for hepatitis B surface antigen (HBsAg). We detected anti-HCV in 2.1% of the blood donors. No statistically significant difference was found between males and females or between locations, but there was a statistically significant increasing likelihood of anti-HCV prevalence with increasing age. HBsAg was found in 8.8% of the 3839 tested donors. There was no statistically significant difference between sexes or age groups, but there was a statistically significant higher prevalence in the islands of Sulawesi and eastern Indonesia. Only 7 individuals, from 5 locations, were both anti-HCV and HBsAg positive. Based on responses to a questionnaire, a history of surgery, blood transfusion, intravenous medication, and acupuncture were identified as risk factors for the presence of anti-HCV. No such risk factor was identified for HBsAg prevalence. The combined data suggest separate modes of transmission for the 2 viruses, and indicate the need for continued surveillance for these agents in Indonesian blood banks.