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IN a previous study of the problem of the inheritance of resistance to bacterial infection in animal species (Bradford Hill, 1934) it was suggested that in endeavouring to breed a resistant strain, and in measuring the average degree of immunity attained by any generation, it might be more profitable to work with a bacterial toxin rather than with a living bacterium. Any experiments in this field require that the animals employed should not be naturally infected with the bacterial parasite against which resistance is to be developed. If they are, the well-known immunizing effects of non-fatal infections will come into play in an unknown and uncontrollable fashion, and will make it difficult or impossible to distinguish clearly between an increased average resistance due to this factor and a similar increase resulting from selective breeding. This difficulty will become greater if, after starting with an uninfected stock, we infect some animals with living bacteria and breed the next generation from the survivors, breeding from the uninfected stock to obtain our unselected controls. Many of the survivors to infection will remain infected, and infective, for a considerable time. Many of them, too, will have developed protective antibodies, and these antibodies the females will transmit to their young, transplacentally or in their milk, giving rise to a transient congenital passive immunity. Since a proportion of the young are likely to become infected from their infected parents, this temporary passive immunity may be followed by a persistent active immunity. The controls, on the other hand, bred in each generation from non-infected parents will not be subjected either to passive or to active immunization. Apart altogether from genetic factors the offspring of parents selected by survival to experimental infection with living bacteria are, therefore, likely to show a higher average resistance than the offspring of unselected and uninfected controls. It is true, of course, that the intervention of passive or active immunization in this method of selective breeding can be tested in various ways. Such tests indicate very definitely that at least a large part of the increased resistance

The inheritance of resistance, demonstrated by the development of a strain of mice resistant to experimental inoculation with a bacterial endotoxin