Antimicrobial susceptibility testing of the fastidious species Bordetella pertussis and Bordetella parapertussis is not standardized. In an attempt to find the optimal medium for agar dilution testing, the activity of erythromycin against Bordetella pertussis and Bordetella parapertussis (34 isolates each) was assessed using homologous broth/agar combinations of Bordet-Gengou, charcoal, Iso-Sensitest (Oxoid) and Mueller-Hinton media. Each medium was supplemented with 5% and 20% whole defibrinated horse blood. Mueller-Hinton medium supplemented with 5% horse blood performed best overall.