1. Vaccines consisting of formalinized Rickettsiae of Mexican typhus fever, obtained by our X-ray rat method, produce definite resistance in guinea pigs to subsequent infection with the virus of this disease.
2. The resistance so produced amounts to complete immunity when the subsequent infectious dose is moderate—that is, consists of typhus blood or of tunica material in reasonable amounts (not more than one-quarter of a tunica—i.e., roughly 100 to 250 infectious doses). When, as in the first experiment, excessive doses of infectious material were given, the vaccination protection was, in two of the three animals, incomplete.
3. Subcutaneous vaccination is fully as effective as intraperitoneal —even when the subsequent infection is intraperitoneal.
4. As in previously reported experiments, the vaccines made with the Mexican organisms conferred only partial and feeble protection against the European virus (Breinl strain).