A study of the effect of graded immunizing doses of typhoid vaccine and graded challenge doses of S. typhosa in 5 per cent mucin on the degree of gradation of response (survival or death) elicited in successive groups of mice is reported.

In the range of doses employed the effect of graded immunizing doses was markedly greater than was the effect of graded challenge doses. Statistically the difference exceeded the 0.1 per cent level of significance.

It was concluded that the use of graded immunizing doses was preferable to the use of graded challenge doses as a basis for the immunological assay of typhoid vaccine, since with the former there was obtained (a) a greater significance of differences in response of groups given graded doses even with smaller fold increments in successive doses, (b) a greater slope of the dosage-response curve which permitted estimation of ED50 values with smaller standard errors, and (c) an effect on response apparently less dependent on the sex of mice used.

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