The phylogenetic distribution of the liver protein F antigen

F antigen is a liver protein of unknown function previously thought to be confined to class Mammalia. Using a recently developed radioimmunoassay, we show that F antigen can also be found in classes Aves, Reptilia, Amphibia and Osteichthyes. Ouchterlony gel diffusion (using murine antibodies) gave lines of identity or partial identity between mammalian F antigens, a line of identity between avian and reptilian F antigens (each showing partial identity to mammalian) and no precipitation with fish or amphibian F antigens. All immunopurified F antigens have molecular weights of 40,000-45,000.