Some of the factors influencing the isolation rate of Bordetella pertussis during a whooping cough epidemic in West Glamorgan, Wales, are reported. The organism was isolated from 39% of patients with clinical whooping cough, pernasal swabbing being much more successful than cough plates. Isolation rates were increased in the non-immunized, particularly in the first year of life. Erythromycin and co-trimoxazole significantly reduced the isolation rate of B. pertussis but this did not occur with penicillin. In this study 20% of patients were culture positive 6 weeks after the onset of their infection. It is suggested that the Department of Health and Social Security recommendation of a minimum period of three weeks exclusion of children from school is inadequate. During the epidemic, the proportion of strains of B. pertussis containing antigen 2 more than doubled.