An experiment recently described in this Journal (Greenwood, Newbold, Topley and Wilson, 1926) dealt with the after–life of mice that had been previously exposed to epidemic pasteurellosis, and led to the following deductions:

“We think we have proved—

(1) That the survivors of herd-exposure to epidemic pasteurellosis are more resistant to subsequent exposure than healthy animals not previously exposed to risk.

(2) That this superiority is significantly correlated both with length of previous exposure independently of its severity, and with severity of previous exposure apart from its duration.

We think it is probable that—

(3) The severity of the prior exposure as measured by the average deathrate during the period of exposure, is less important than the length of exposure.

(4) The advantage of exposure at first increases with its duration and then decreases, so that mice who have been exposed for a moderate time are more resistant to subsequent exposure than mice who have been exposed for a very short or a very long time.”

The results of an independent experiment, similar in kind, with the difference that the after-life and previous exposure took place in a single cage, and covering 1867 mice, support(1), (3) and (4) of the above deductions, also (2) so far as it relates to length of previous exposure, but the severity of previous exposure apart from its duration shows a negative but insignificant association with improved expectation of after-life. These results are in agreement with the suggestion inspired by the experiment previously described. “That these facts are difficult to interpret in terms of pure selection and more easily reconciled with a process of active immunization during the primary exposure, but that the nature of this process will remain obscure until we have more experimental data at our disposal.”