Abstract
A nonlethal strain of Rickettsia tsutsugamushi was separated from chiggers, Leptotrombidium pallidum. Experimental infection with a nonlethal strain, as well as with a lethal strain, induced protection against the lethal strain of rickettsia (Gilliam) in mice. Serum antibody titer was not augmented by the challenge infection in any case. Antibodies are probably less important in protection against the challenged rickettsiae.
