beta-Lactamases from five strains of Bacteroides fragilis and two strains of Bacteroides uniformis, all resistant to beta-lactam antibiotics, were compared by means of isoelectric focusing and enzyme kinetic measurements. beta-Lactamases from the five B. fragilis strains were identical, whereas those from the two B. uniformis strains were distinguished from each other and also from the B. fragilis enzymes. The two B. uniformis strains were relatively resistant to cefoxitin (minimal inhibitory concentration, 32 micrograms/ml), but only one of the strains, B. uniformis 2986, was found to rapidly inactivate cefoxitin. This apparently enzymatic inactivation of cefoxitin seemed to be of minor importance, and the main factor for cefoxitin resistance was considered to be a decreased permeability of the drug. Transfer of resistance to beta-lactam antibiotics from these beta-lactamase-producing strains of B. fragilis and B. uniformis to a non-beta-lactamase-producing strain of Bacteroides distasonis was attempted, but with these isolates no transfer of resistance to beta-lactam antibiotics was demonstrated.