The author investigated the development of neutralization antibodies immunized with virulent rickettsiae, and also tried the investigation of differences of antigenicities among the rickettsiae by cross neutralization tests. Two groups of strains were used for these experiments, one group is three newly isolated strains-two strains (No. Ⅳ and ⅩⅩⅡ) of original summer type of tsutsugamushi disease from rats and one strain of the winter type of the same disease from patients, and the other group is the three known control strains-original summer type (Oseki), winter type (Osawa) and Carp (New Guinea). 1) The maximum production of neutralization antibodies in the immunized rabbits was observed five to seven weeks after the inoculation for the group of newly isolated strains, and ten weeks for the group of the known control strains; the neutralization logarithm for the former was 2.2 to 2.6 and that for the latter was 4.5 to 60. 2) By cross neutralization tests, fairly clear differences of type were observed among the newly isolated strains, and also among the known control strains. It was sometimes very hard, however, to divide the whole of the newly isolated and known control strains into several groups according to the type differneces.