1. Titration and neutralization of the Western strain of equine encephalomyelitis virus can be carried out in vitro by means of tissue culture.

2. The in vitro titration test as presented is a more sensitive method than animal inoculation.

3. Tissue culture may be better than animal inoculation for the detection of small amounts of virus.

4. The neutralization obtained in tissue culture is 100 to 1,000 times greater than that observed in the intracerebral test in mice and is comparable to the potency obtained by the intraperitoneal route of inoculation.

5. The possibility of applying this method to the study of other viruses, known and unknown, is discussed.

6. Further applications of the method described above to evaluation and standardization of bacterial toxins and antitoxins and to the testing of the toxicity of biological and chemical products are suggested.

7. Evidence for the reactivation in vitro as well as in vivo of virus in neutral mixtures by dilution is presented.

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