The adenylate cyclase toxin (ACT) of Bordetella pertussis has been shown to penetrate eucaryotic cells and produce a rapid elevation in intracellular cAMP which leads to altered cell function. Recent studies have demonstrated an intracellular state for the bacteria within professional and non-professional phagocytes. A virulent strain was compared to two ACT defective strains to determine if this toxin contributes to intracellular survival within human macrophages. When challenged by 10(6) macrophages/ml in a cell invasion assay, 10(3) bacteria/ml were recovered from samples containing the ACT defective strains. These values were two log units less than the number of bacteria recovered from samples containing the isogenic parent. The binding and uptake of all strains by the macrophages were equivalent, suggesting that ACT does not affect adhesion nor endocytosis but rather protects against macrophage killing following uptake. Drug-induced elevation of cAMP levels within the macrophage by forskolin increased the number of surviving bacteria in samples containing the mutant strains to values equal to those obtained with the parent strain. Therefore, the protective effect conveyed by ACT is the result of toxin-induced elevation of cAMP within the macrophage concomitant with bacterial uptake.