The authors made a comparative study on the value of the Weil-Felix reaction and the Fixation of the Complement in the diseases of exanthematic typhus group. They come to the conclusion that both have qualities and defects. In precocius disease, both failed. in the period of development of the disease, two sorological tests often produce decisive results. From the fiftieth day of the natural disease, the reaction of the fixation of the complement is more precise, in the investigation of former cases of individuals, free from constant and repeated injection of virus by the hematophagous carriers. In those patients that remain in the known centres of the disease, subject to constant inoculations of the virus, the Weil-Felix is also of great diagnostic value. The authors showed that the sheep (Ovis avies) is only slightly sensitive to the V.B. strain of Brazil and that the armadillo (Tatus novencinctus) is not sensitive to the same strain. Repeating former data, verified in Bello Horizonte, the authors found that the V.B. strain of the brazilian virus goes through the placenta and infects the organism of the fetus. They verified the presence of the murine variety of Exanthematic Typhus in Ouro Preto and other cities of Minas Gerais.