We have studied serological markers of viral hepatitis type A, B, delta and C in 7,713 blood-donors, 265 patients with a clinical diagnosis of hepatitis, 41 inmates of a mental and physical retardation institution and 41 health care workers at the same institution and 35 chronic hemodialysis patients. The results showed a 0.8% anti-HCV prevalence in blood-donors, but a higher percentage (47.5%) among HBV positive patients and in two different groups of the inmates at the mental institution (12.5% and 36%). We can not establish a relationship with the presence of anti-HCV and serological markers for HBV or H-delta V, neither with any serological markers pattern of B hepatitis nor with the anti-HCV levels and pathological findings in the biopsy of the cases in which this procedure was performed. The detection of anti-HCV has two direct applications: to exclude positive blood-donors in order to reduce the risk of post-transfusion hepatitis and to better establish a precise diagnosis in patients otherwise classified of having nonA-nonB hepatitis.