Identification of the leptospira isolated from a case of yellow fever in Merida was accomplished by means of an anti-icteroides immune serum prepared in a horse with several Guayaquil strains of Leptospira icteroides. The immune serum showed a protective action of high titer against the Merida strain, thus establishing its efficacy as a therapeutic agent against this strain. Polyvalent anti-icteroides immune serum prepared in the horse or monovalent anti-icteroides immune serums prepared in the rabbit had a definite devitalizing action upon the Merida strain, while immune serums similarly prepared with strains of icterohœmorrhagiœ had no perceptible effect upon the Merida strain.

Serums from yellow fever convalescents in Merida gave a positive Pfeiffer reaction with the Merida strain of Leptospira icteroides. The reactions between the Guayaquil strains (Nos. 1 and 5) and two of these serums from convalescents varied from definitely positive to doubtful, owing probably to the diminution of active immune principles in the serums during the prolonged and unfavorable conditions of their transportation.

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