AbstractSusceptibility of anaerobic clinical isolates from the United States, Canada, and Germany to sulbactam/ampicillin (1 + 2) (SBT/ AMP), ampicillin alone (AMP), and cefoxitin (CFX) was determined with a standard agar dilution test. The isolates included 192 strains of Bacteroides fragilis, 132 strains of other Bacteroides spp., and 19 strains of Clostridium spp. Against all species tested, SBT/ AMP was more active than AMP or CFX Results obtained by a new model of associative susceptibility testing indicated that susceptibility testing of single pathogens from polymicrobial infections is not necessarily reflective of the susceptibility of the pathogens at the site of infection. The results suggest that sulbactam is effective in associations of pathogens producing beta‐lactamases of Richmond‐Sykes types II–V and beta‐lactamases of anaerobic bacteria.