The author reinvestigated the procedures of neutralization test of Rickettsia tsutsugamushi in regard to the contact period of the agent with immune sera. At the same time, the "Absaettigungsversuch" (Wildfuehr) was also tried on R. tsutsugamushi. Mitani strain of R. tsutsugamushi isolated from the patients of Umayado disease in Kagawa prefecture was used as the test agent. The results were as follows: 1) In test tubes, R. tsutsugamushi combines with its antisera in a short time, and the virulence of R. tsutsugamushi falled along with prolongation of contact period. These facts suggest that estimation of the true titer of neutralization antibody become possible by the short-period-contact which causes no noticeable fall of rickettsial virulence. 2) In the neutralization test of R. tsutsugamushi, dilution of antisera to be mixed with rickettsiae in test tubes deteriorates its result, and addition of human or guinea-pig sera had no influence on its neutralizing effect. 3) No noticeable result was obtained in application of the "Absaettigungsversuch" to R. tsutsugamushi.