The transfer of neutralizing antibody to calves born of dams vaccinated against foot-and-mouth disease was by colostrum only. Immunoelectrophoretic study showed that calves were born with no γ-globulin in the serum, but that it was present 2 hr after they ingested colostrum. Transfer of neutralizing antibody could be blocked by prior feeding of skim milk or immune bovine serum. A passively immune calf did not respond to vaccination until the serum antibody reached low levels; whereas, a calf of the same age born of a nonimmune dam could be vaccinated as evidenced by the production of neutralizing antibody.

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