A group of sera from autochtonous cases of Brill-Zinsser's disease, in the early acute phase, were examined by the tests of Murray et al. and Voronova, in the course of a study on the characteristics of the immune response in primary and secondary stimulation with Rickettsia prowazeki. The results obtained both with these sera and those of primary typhus cases from other countries, suggest the possibility of establishing a serologic diagnosis of Brill-Zinsser's disease, with certainty, by identifying the secondary nature of the disease according to the presence of antibodies type 7 S. The authors recommend complement fixation with increasing soluble R. prowazeki antigen concentrations as a method of electron for routine diagnosis.