Seventy-one strains of species from the Bacteroides fragilis group, including 46 isolates of B. fragilis, were tested for susceptibility to the bactericidal effect of serum from healthy subjects. Twenty-seven (38%) of the isolates were killed by serum. Isolates from feces were significantly more sensitive to serum than were isolates from patients with clinical infections. Killing of bacteria required heat-labile serum components and was an exponential function of serum concentration. Among the various species tested, B. fragilis was clearly the most resistant to bactericidal activity of serum. These observations may be important to the understanding of infections caused by the B. fragilis group, which contains the anaerobes of greatest clinical importance.