Posttransfusion hepatitis remains a threat to transfusion therapy. Testing for increased ALT levels has been used in an attempt to reduce this risk. Presence of the infectious agent, hepatitis C virus (HCV), appears to be a much more sensitive criterion. Stored serum samples from transfusion blood as well as recipients of transfusion were tested by ELISA, RIBA and PCR for the presence of HCV. The results show that RIBA and PCR are about equally sensitive and are able to detect HCV positivity in many sera that might have been otherwise transfused. Routine screening for the presence of virus will dramatically reduce the danger of hepatitis infection to transfusion patients.