We have been able to confirm all the experimental results of Felix and his collaborators in regard to the Vi antigen and its significance in mouse infections. In the interpretation of some of the findings we are, however, not quite in agreement with Felix.

When large doses of living V forms of typhoid bacilli are injected into the mouse they cause acute intoxication followed by death within 2 days—if the animals die. Surviving animals are clinically healthy after this time, often earlier, but they are nevertheless found to be massively infected in the organs and blood for several days.

When smaller, non-intoxicating, doses are used it is not possible to demonstrate any difference between the V form and the W form of the same strain in regard to the severity of the infection they cause; so that we may conclude that the Vi antigen plays no part in the virulence of the typhoid bacillus in the mouse. (Remember in this connexion that the dysentery bacillus is also very toxic for mice injected in large doses, while its virulence, i.e. the ability to grow in the mouse is = 0; but when rapidly fatal doses are used a massive growth of dysentery bacilli can also be demonstrated here, the cause being that the natural immunity has become partly neutralized by the intoxication.)

Like Felix we find that only the V form can be used both for active vaccination and the production of sera which protect against lethal doses of V forms in mouse experiments. We consider that this is due to the antitoxic effect. Our experiments show that the anti-bacterial effect in vaccination experiments differs little whether the V or the W form is used as vaccine, so long as the vaccinal effect is tested with non-toxic doses of bacilli. As shown in earlier experiments, the bactericidal effect in actively vaccinated animals is often a relatively slow process.

Only the Vi antibody containing sera can protect prophylactically with certainty against the toxic effect of living typhoid bacilli. On the other hand; the bactericidal effect prophylactically of 0 and 0 + Vi sera is very similar when tested against non-toxic doses of “virulent” typhoid bacilli.

The therapeutic value of potent Vi sera under the experimental conditions given was nil.