X. Formolized vaccines obtained (with Mexican organisms) from rats in which resistance to typhus had been depressed by exposure to radiation with short-wavelength x-rays contained a much higher concentration of Rickettsias than any others hitherto employed by the authors. These vaccines produced definite resistance in guinea pigs, amounting to complete immunity when the subsequent infectious dose was moderate. Only partial protection was induced against European virus (Breinl strain).[long dash]XI. Serum from horses treated with formalin-and phenol-killed Rickettsias obtained from Mexican virus by x-ray rat technique exerted distinct protective action against the same virus, whether mixed with virus before injection, administered 1 wk. or longer before the virus, or given 24, 48, or 72 hrs. after the virus. This serum also agglutinated the Weigl louse vaccines containing Rickettsia prowazeki of the European disease in dilutions comparable in potency to the Weil-Felix reaction.