Ninety-six strains of bacteroides were tested for the presence of β-lactamase activity using the chromogenic cephalosporin nitrocefin, an iodometric technique for the detection of penicillinase, and a penicillinase disc test employing a bromcresol purple pH indicator. Fifty-three strains of Bacteroides ruminicola ss brevis exhibited an excellent correlation between resistance to β-lactam antibiotics and detection of β-lactamase by all three methods. Among 40 resistant strains of the Bact. fragilis group, 78% could be shown to produce β-lactamase with nitrocefin. However, the two tests for penicillinase production were positive in only one strain each. These methods of rapid β-lactamase testing cannot be used reliably to predict the susceptibility of unidentified Gram-negative bacilli to penicillin. However, a positive nitrocefin slide test accurately predicted resistance to penicillin and cephalothin in our study.