-Highest foetal abortion (i.e., 98.4%) was obtained for inoculation of B. abortus 544 on day 4 p.c. Infection with B. abortus ΔvirB4 or S19 strains resulted in no abortions.-B. abortus 544-infection induces IFN-γ production, with a peak at 3 dpi, that in pregnant animals was associated with abortion induction. -IFN-γ production did not happen in ΔvirB4- or S19-infected mice.-Brucellae were isolated from both aborted and alive foetuses or placentae at similar rates.-Brucellae were present within parietal TGCs, neutrophils, or free at the decidua. This evidence concerns the gene IFNG and abortion.