An abrupt decrease in the titer of serum T-agglutinin frequently occurs during the acute phase of anthrax infection in the rabbit. In partially immunized animals which survive the infection the titer returns to normal during convalescence, but has not been observed to rise significantly above the normal level.

The presence of a substance capable of inhibiting T-agglutination may be demonstrated in the sera of many of those animals in which a marked decrease in T-agglutinin has occurred. The polysaccharide and polypeptide antigens of B. anthracis do not possess this activity.

Sera containing the T-inhibitory substance usually produce a slow hemolysis of T-erythrocytes and also of normal human type O erythrocytes; hemolysis of erythrocytes of other species is irregular. This reactivity of the sera withstands inactivation at 56°C. but is inhibited by citrate.

T-inhibitory and hemolytic activities can frequently be increased by incubation of the acute-phase serum at 37°C.

The ability of normal serum to neutralize the T-inhibitory and hemolytic activities of acute-phase serum is not significantly increased after recovery from the infection.

The meaning of the results is discussed.

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