The measurement of neutralizing antibody response to smallpox vaccination by various test systems has been unsatisfactory because small amounts of residual, uncombined virus can multiply and obscure the result. This can be avoided by use of a tissue culture plaque technique which immobilizes the spread of residual active virus. It has been found that a successful primary vaccination elicits a uniformly good response as measured by both neutralization and hemagglutination-inhibition. In

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... revaccination provokes a greater elevation in the neutralizing titer and this rise appears to be related to the circulating antibody level at the time of revaccination.