Samples from a frozen plasma pool and frozen serum specimens from serum hepatitis studies conducted between 1951 and 1954 were tested for hepatitis-associated (Australia) antigen (HAA). The incubation period of clinical hepatitis and the time of appearance of HAA in recipients varied inversely with the dose of virus inoculated using a series of dilutions of the icterogenic pool of human plasma which contained HAA. By assuming an analogous relationship of incubation time of serum hepatitis and HAA to dose of virus inoculated, it was possible to estimate the relative infectious virus content of other materials used in the same studies, including treated fractions of the plasma pool, derivatives of the plasma pool, and serum from hepatitis carriers and an icterogenic lot of thrombin which were unrelated to the plasma pool.