There is evidence that transmission of serum hepatitis is associated with transmission of virus-like particles, approximately, 20 millimicron in diameter, containing the Australia or serum hepatitis (SH) antigen, which is currently referred to as the hepatitis associated antigen (HAA). Virus-like particles containing HAA were in the following materials, inoculation of which produced serum hepatitis: (1) a pool of human plasma, (2) serum obtained during the acute phase of hepatitis from a recipient of the plasma pool, (3) a preparation of human thrombin, and (4) serum from a proved hepatitis carrier. The HAA appeared in the serum samples of 61 individuals inoculated with these materials; serum hepatitis developed in 38 of them. Inoculation of dilutions of the plasma pool showed that serum hepatitis can be transmitted by materials containing HAA in amounts too low to be detected by current techniques.