Although ceftizoxime has been used effectively in several clinical trials for infections involving anaerobic bacteria, reports of its in vitro activity against anaerobes are contradictory and confusing. In an effort to clarify the discrepant reports, we tested 90 strains of Bacteroides fragilis group organisms from patients with perforated or gangrenous appendicitis using eight different susceptibility testing procedures. The minimal inhibitory concentration values were dependent on the technique used; agar dilution values were often four twofold dilutions higher than microbroth dilution values. Agar techniques (including spiral gradient end point) gave values of 36% to 61% susceptible at breakpoint (depending on the technique), while the microbroth dilution techniques gave values of 84% to 92% susceptible. When a 64 microgram/ml breakpoint for agar dilution testing was used, the methods were more comparable, with the agar methods giving values of 64% to 83% susceptible. The results of the broth disk elution procedure were difficult to read and often did not agree with other values.