The α-fetoprotein in fetal and newborn sera of ten mammalian species was studied immunochemically and electrophoretically.
Reaction of the sera of fetal horses, cows, sheep, pigs, rabbits, dogs, cats, rats and mice with horse antiserum against human α-fetoprotein were studied. All sera except those of rats and mice gave variable amounts of immune precipitate with rabbit and rat antisera to human α-fetoprotein. On the other hand, horse and rabbit antisera against rat α-fetoprotein reacted only with rat and mouse α-fetoprotein but did not react with the α-fetoprotein of human or of the other species. Thus, antigenic differences of the α-fetoproteins of these species were clearly demonstrated.
Horse α-fetoprotein precipitated with horse antisera against human α-fetoprotein, and rabbit antisera to human α-fetoprotein also reacted with rabbit α-fetoprotein. Rats did not produce any precipitating antibody to rat α-fetoprotein after extensive immunization. The above results were discussed in reference to loss of immune tolerance.
The various α-fetoproteins migrated electrophoretically between albumin and β-globulin, and mobility differences among species were noted.

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