Concanavalin A (Con A), a lectin isolated from jack beans, binds specifically to saccharides with terminal α-d-mannopyranosyl, α-d-glucopyranosyl or β-d-fructofuranosyl residues (Goldstein, Hollerman & Merrick, 1965a; Goldstein, Hollerman & Smith, 1965b; Goldstein & So, 1965). Glycoproteins, present in human serum (Leon, 1967; Morse, 1968) or representing structural components of membranes of mammalian cells (Inbar & Sachs, 1969) or of enveloped viruses (Oram et al. 1971; Becht, Rott & Klenk, 1972; Calafat & Hageman, 1972; Klenk, Rott & Becht, 1972) react with Con A. Recently, Cawley (1972) observed that Con A partially precipitated hepatitis B antigen. Results presented here extend this finding and show that the interaction between hepatitis B antigen and Con A may be utilized as a step in purifying the distinct morphological forms of this antigen.