1.It has been shown that between 18 and 21 days of age, mice are intermediate between new born and adults in their susceptibility to the intraperitoneal injection of yellow fever virus. Their degree of susceptibility is such that uniform death may be produced by injecting one-tenth of the virus dosage required for adult mice without the need for the additional intracerebral injection of starch. The shift in susceptibility at this age is rapid so that it requires three times as much virus to kill the 21-day-old mice as it does to kill 18-day mice.
2.Although at any one age the smaller mice are more susceptible to virus than the larger, the difference is less than observed between mice of different ages. Small 21-day-old mice are more resistant to virus than 18-day-old mice even though the latter outweigh them. There is no evidence that sex influences the level of immunity at any age.
3.Eighteen- to 21-day-old mice injected with immune serum-virus mixtures are as readily protected against virus as are adult mice. When two parts of immune serum are added to one part of virus, the results of injecting 0.06 cc. of the mixture into 18- to 21-day mice are equivalent to injecting adult mice which have received intracerebral starch with 0.6 cc. This permits satisfactory tests on much smaller serum samples than required for the standard test.
4.By increasing the proportion of immune serum to virus and increasing the volume of inoculum so that the amount of virus received by each mouse remains constant, the protective capacity of the immune serum is augmented. Because of this the test in young mice has been demonstrated to be of value in the detection of small quantities of antibodies.
5.An analysis of the results of a series of routine intraperitoneal virus and immune serum titrations in young mice indicates that there is a more rapid transition from protection to no protection as the quantity of immune serum is reduced than from death to survival as the quantity of the virus is reduced. As individual variation in mouse susceptibility operates at its maximum in this transitional zone, immune serum titrations are less variable than are virus titrations.
6.The technique of the test is given in the appendix.