There is considerable data to support the hypothesis that Australia antigen is an infectious agent that causes hepatitis in man.

(a) Association with acute viral hepatitis.

(b) Association with chronic hepatitis.

(c) Virus-like appearance under the electron microscope (200-A particles).

(d) Transmission of Au(1) from man to man.

(e) Transmission and passage of partially purified Au(1) to an animal host (infant African green monkey).

(f) Localization [with fluorescent anti-Au(1)] of Au(1) in the nuclei of liver cells of patients with hepatitis and/or Au(1) in their blood.

(g) Distribution of Au(1) in institutions, disease groups, and populations is consistent with the distribution of an infectious agent.

(h) RNA identified in Au(1) particles isolated from blood.

(i) Apparent replication of Australia antigen in tissue cultures of human liver cells.

There is also considerable evidence that Australia antigen has many of the characteristics of a serum protein polymorphism. Since neither of these hypotheses has been rejected they can be combined to make a third hypothesis, namely, that Australia antigen is an infectious agent which causes hepatitis in some people infected with it and that it has the characteristics of an (inhertied) serum protein polymorphism. We propose calling agents of this postulated class "Icrons."