The complement fixation test as applied to infectious abortion is specific, and serves as a valuable method of diagnosis.

With the partial revision and simplification of technic presented in this paper the method should be thoroughly practical and reliable.

The most satisfactory antigen was obtained on nutrient agar containing Fairchild peptone, and having an initial hydrogen ion concentration of PH 6.8.

The incubation period of the B. abortus cultures should not exceed four to five days, and the stock antigen suspensions should be prepared immediately following the removal of the culture tubes from the incubator.

Antigen suspensions with a turbidity of 1.75 in terms of the McFarland nephelometer lend themselves readily for direct antigen titration.

The Wenner method of bleeding guinea-pigs, with the present refinement, is a very practical and economic one, and can be mastered readily by the ordinary operator.

Complement stabilized with 40 per cent of a 12 per cent sodium acetate solution retains its complementary properties for three to four weeks.

Formalinized sheep's blood may be used for three to four weeks as the immediate source of hemolytic antigen in the fixation test. Freshly-washed corpuscles from formalinized blood may be used also as antigen for hemolysin production.

Positive and negative sera should be used as controls in the final fixation tests.

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