It has proved possible to elicit passive immunity to B. typhosus reacting factors by means of normal and immune homologous neutralizing antibodies. The in vivo serum protection against these factors followed the law of multiple proportions.

There was observed a considerable loss of antibodies from the blood stream.

Passive immunity was best obtained when the immune serum was injected intravenously ½ hour before the intravenous injection of the reacting factors.

It was possible to prevent the occurrence of the local skin reaction by an intravenous injection of serum after the intravenous injection of the reacting factors, provided the serum dose was very large and provided the serum injection was made immediately after the filtrate injection.

A number of experiments clearly demonstrated the interesting fact that the greater the amount of antiserum injected intravenously, the more efficient was the in vivo neutralization, in a ratio distinctly greater than the quantitative increase of serum. It is suggested that there may be a practical value of the observation in relation to serum therapy.

The results also demonstrated passive serum protection against the lethal effect of B. typhosus "agar washings" filtrates, in a ratio which seemed to suggest the law of multiple proportions.