The detection of Australia antigen (HBsAg) in blood donors and of the corresponding antibody, has become a routine practice in blood banks and transfusion services. They are the best possibilities currently available in the laboratory to exclude, as blood donors, the apparently healthy antigenemic individuals, whether carriers or if incubating a type B, viral hepatitis and the individuals with a past (probably subclinical) immunological experience to virus type B, as indicated by the presence of specific circulating antibodies. The fact that both hepatitis virus, A and B, may be transmitted by different routes and mechanisms, implies the risk of post-transfusion hepatitis, particularly when the infected person is asymptomatic or becomes a "healthy carrier, and donates blood. It is important to emphasize that even the most sensitive techniques currently in use are unable to detect more than 50% of hepatitis B-infected persons. They must be considered, therefore and so far, screning procedures with clear limitations. The detection of antibodies against virus type A has only very recently been accomplished, but the availability of standardized antigen is limited to a few laboratories at the present time. Research on a diagnostic test for virus A, equivalent to that of Au antigen, has proven unsuccessful up to now. The Virus Laboratory of the Hospital del Nino I.M.A.N., using the Cep method, has detected 31 HBsAg positive sera out of 6,372 voluntary blood donors attending the hospital's blood bank in the last 4 years. The resulting frequency of 4.86/1,000 is probably below the figure that might be obtained by applying more sensitive methods, periodical sampling and additional tests such as those related to liver function.