Data indicating that certain strains of Rickettsia tsutsugamushi show definite differences from one another when examined by means of cross-vaccination tests were presented in a previous paper of this series (1). This article also summarized the results of serological studies by other workers which support the idea of heterogeneity within this species. The present investigation, which was undertaken concurrently with the cross-vaccination studies mentioned above, was designed to extend earlier observations made in this laboratory (2) on the relation between strains of R. tsutsugamushi as revealed by cross-neutralization tests.

Material and Methods. Strains of R. tsutsugamushi.—Ten different strains of R. tsutsugamushi were employed in the present study. These had been recovered from man, mites, or rodents in widely scattered areas of the South Pacific and Orient. The species of host, the geographic area from which each of the strains was obtained, and the date of isolation are shown in table I.

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