Sixteen Bifidobacterium isolates from the human gastrointestinal tract were assayed for susceptibility to 44 antibiotics by soft agar overlay disc diffusion on TPY agar. Five isolates (3/7 B. bifidum and 2/3 B. breve) exhibited atypical antibiotic susceptibility profiles. Poor growth in the agar overlay accounted for susceptibility of B. bifidum but not B. breve isolates. All other isolates were resistant to cefoxitin (30 μg), aztreonam (30 μg), vancomycin (30 μg), amikacin (30 μg), gentamicin (10 μg), kanamycin (30 μg), streptomycin (10 μg), fusidic acid (10 μg), trimethoprim (5 μg), norfloxacin (10 μg), nalidixic acid (30 μg), metronidazole (5 μg), polymyxin B (300 μg) and colistin sulphate (10 μg), and they were susceptible to the six penicillins studied, cephalothin (30 μg), cefuroxime (30 μg), cefaclor (30 μg), ceftizoxime (30 μg), cefotaxime (30 μg), bacitracin (10 μg), chloramphenicol (30 μg), erythromycin (15 μg), clindamycin (2 μg), rifampicin (5 μg) and nitrofurantoin (300 μg). In addition, they varied in their susceptibility to cephradine (30 μg), cephazolin (30 μg), cefoperazone (75 μg), ceftriaxone (30 μg), ofloxacin (5 μg) and furazolidone (15 μg). They were resistant, or only marginally moderately susceptible, to ceftazidime (30 μg), netilmicin (10 μg), sulphamethoxazole (100 μg), cotrimoxazole (25 μg) and ciprofloxacin (5 μg), and susceptible or marginally moderately susceptible to tetracycline (30 μg). All B. bifidum isolates were susceptible to cefixime (5 μg). Four micro-organism-drug combinations were evaluated for β-lactamase activity but its absence suggested that cell wall impermeability was responsible for cephalosporin resistance among bifidobacteria. The antibiotic susceptibility of B. animalis 25527T was similar to that of the human isolates.