In this study, Tanizawa, rat IV, and rat XXII strains, which were isolated in Kagawa prefecture, and the hitherto isolated Shichito and Karp strains were used. Each fraction was prepared from the eggs successively inoculated with the agents into yolk sacks. The results are as follows: 1) All of the yolk, yolk sack and brain of embryo, prepared from the non-inoculated healthily growing egg, do not cause any hemagglutination. 2) The yolk of the infected egg, regardless of the sorts of inoculated strains, does not cause any hemagglutination. 3) Within 5 hours after the preparation, the emulsion of yolk sack of infected egg causes remarkable hemagglutination. The suspension of purified rickettsiae, however, causes more remarkable hemagglutination, and the rest of yolk sack tissue, from which the rickettsiae were purified, has no hemagglutination activity. 4) Regardless of the sorts of inoculated strains, the yolk sack loses its hemagglutination activity by aging at 4°C for more than 7 days. 5) The emulsion of yolk sack loses its hemagglutination activity by treating it with ether for a long time. 6) In short, the yolk sack emulsion of infected egg has the hemagglutination activity as far as it is fresh, but loses it by aging. This hemagglutination activity is preserved not in the yolk sack tissue, but in rickettsia itself, and is attributable to a ether-labile factor which does not bear on the sero-immunological antigenisity.