Anaerobic bacteria in the lochia are frequently isolated healthy post-partum patients and from those with post-partum infections. The true pathogenic significance of these anaerobic bacteria is uncertain. Investigations appeared to show that the incidence of anaerobic bacteria is by and large the same as in the vaginal and intestinal flora. In this investigation we showed: 1) anaerobic bacteria are rare in the vaginal flora of pregnant patients in the third trimester, 2) a definite difference exists in the number of anaerobic bacteria in normal lochia versus infected lochia, 3) in post-partum infections qualitative differences exist between the anaerobic isolates. The strains of the bacteroides oralis group become the predominant anaerobic bacteria. It is concluded that members of the bacteroides oralis group which are normally not present in the human intestinal flora play a specific role in post-partum infections. The anti-microbial treatment of post-partum infections has to take this into account. The eminent therapeutic importance of the 5-Nitro-Imidazoles is discussed.