In a field trial conducted in cooperation with pediatricians in private practice, 198 nasopharyngeal swabs from children with suspected whooping cough were placed into charcoal-horse-blood transport medium with cephalexin (40 mg/liter). After preincubation at 36 degrees C for 1 to 2 days, the transport systems were mailed to the laboratory. There, the swabs were plated onto charcoal-horse-blood agar with cephalexin and were subsequently incubated for 48 h in cephalexin-containing charcoal-horse-blood broth which was then subcultured onto the agar. Forty-six Bordetella pertussis strains and seven Bordetella parapertussis strains were isolated (Bordetella isolation rate, 26.8%). Only three (5.7%) of the 53 Bordetella strains were detected exclusively by use of the broth. This low rate of additional isolates is probably explained by the fact that the swabs had been submitted in charcoal-horse-blood transport medium which itself acts as an enrichment medium.