The practical application of an initially esoteric finding had come about only two years after the publication of our paper on the association between Au and hepatitis. In retrospect, one of the major factors contributing to the rapid application of the findings was the simplicity of the immunodiffusion test. Another was our program of distributing reagents containing antigen and antibody to all investigators who requested them. We did this until this function was assumed by the National Institutes of Health. Following the confirmation of the association of hepatitis with Australia antigen, a large number of studies were published, and, in a relatively short time, the routine use of the test in blood banks became essentially universal in the United States and many other countries. It has been estimated that the annual saving resulting from the prevention of post-transfusion hepatitis amounts to about half a billion dollars in the United States.