Pertussis toxin is a principal determinant of virulence produced by Bordetella pertussis in the disease whooping cough and is the primary toxinogenic component of the pertussis vaccine. This toxin is not produced by the closely related species Bordetella parapertussis. Toxinogenic strains of B. parapertussis were constructed by the conjugative introduction of cloned pertussis toxin genes. These and analogous toxinogenic and nontoxinogenic strains of B. pertussis were assayed for their toxicity and reactogenic activities. Expression of active toxin by either Bordetella sp. correlated with the induction of leukocytosis, anaphylaxis, and histamine sensitivity.