The sera of guinea pigs infected with or recovered from either variety of typhus (Mexican and European) possess opsonins for Rickettsia prowazeki (Mexican type). The opsonizing properties have been observed in the blood of guinea pigs as early as 96 hours after the inoculation of Mexican typhus virus.

In the convalescent and recovered guinea pigs the opsonizing properties of the serum have been found to persist as long as 3 months after the infection.

The sera from patients and human convalescents from Mexican typhus, possess opsonizing properties for rickettsial bodies. This property is still apparent in the sera of persons who had had typhus or Brill's disease from 2 to 6 years previous to the date of the present studies.

Following vaccination with the formalinized Mexican rickettsiae, opsonins appeared in the blood of a normal man.

Anti-proteus X-19 sera fail to opsonize the rickettsiae of Mexican typhus fever.

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