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The presence of genetic material in Au(1) was first reported by Jozwiak et al (5) in January, 1971. In their studies, nucleic acids were found in small (Juantities (5 percent) in purified Au(1), and colorimetric tests were negative for DNA. They concluded that Au(1) contains RNA; this has been con­ firmed by Millman and colleagues. In an earlier study, Millman et al (6) reported that if RNA were present in Au(1) it would constitute less than 10 percent of the total weight of protein. Australia antigen has been isolated from plasma by pretreatment with a variety of proteolytic and other enzymes, followed by gel filtration and density centrifugation. The isolated particles of Au(1) (free of normal serum constituents) have an electron microscopic appearance similar to that of small viruses (Fig. 1a). When purified Au(1) is extracted with 2 percent Tween 80, the immunoreactivity to anti-Au(1) is destroyed. After this treatment, surface components are absent and coiled strands have appeared (Fig. 1b). This material appears to be mainly protein. The supernatant after Tween 80 treatment contains the following constituents (7): 1. IgG reacting with antisera to both heavy and light chain; 2. a component react­ ing with anti-beta la/1c (presumably C3 complement) ; 3. a component which reacts with anti-human beta-lipoprotein; 4. a component which re­ acts with anti-transferrin; and 5. a substance which reacts with anti-albumin. It thus appears that some of the components of Au(1) are substances similar to human serum proteins. The possibility of these representing serum pro­ teins nonselectively absorbed on the surface is unlikely for the following