The sensitivities of seven methods of enzyme-immunoassay for HBsAg detection, as well as of twelve immunoassays for detecting antibody to HCV, were compared, in an attempt to evaluate the need for an official technical validation of such methods prior to its commercialization in Spain. Significant differences were seen for the sensitivities of these methods, which may influence the proficiency of blood bank screening for HBsAg and anti-HCV for preventing cases of post-transfusional viral hepatitis. As a conclusion, it is recommended to establish legal regulations for pre-marketing validation of such assays in Spain, as well as to take the results obtained in these evaluation studies as the basis for selecting those tests which may be more convenient for screening purposes at the blood banks.