Summary:A prospective investigation of the development of hepatitis and of Australia (Au) antigenaemia was conducted in 30 unselected patients subjected to open‐heart surgery. Eleven (37%) developed post‐operative hepatitis nine of whom were icteric. Six of the eleven had Au antigenaemia.Of the 30 patients five were found to have Au antigenaemia prior to operation and this is considered to be probably the consequence of cardiac catheterisation which, as far as we are aware, has not been previously reported. This renders suspect the efficacy of the method of sterilisation in common practice. Subsequent to operation, three of the five developed hepatitis but during operation all three had been given one unit of Au antigen‐positive blood. In the remaining two, Au antigen was not detected in the post‐operative period, conceivably the result of the dilution effect of multiple transfusions. Of four other patients given Au antigen‐positive blood one developed hepatitis but three others developed antibody to Au antigen. This latter finding is considered to be an anamnestic response and is further evidence of the high frequency of infection by the Au antigen‐associated virus in the community. The source of the Au antigen in the remaining two patients who developed hepatitis could not be