Pertussis toxin (PT) has been shown to act as an adjuvant that enhances the production of both Th1 and Th2 cytokines to coinjected protein antigens. It has remained unresolved, however, how PT affects the clonal sizes, long‐term effector functions, and Th1/Th2/Th0 differentiation of the T cell responses induced. We have studied the effects of PT on the development of the CD4⁺ T cell response to a prototypic antigen, hen eggwhite lysozyme (HEL). HEL injection with incomplete Freund's adjuvant (IFA) resulted in an IFN‐γ^–/IL‐5⁺ Th2 recall response. In comparison, co‐administration of PT with HEL:IFA enhanced the frequencies of IL‐5‐producing T cells up to eightfold, and induced the differentiation of high frequencies of IFN‐γ‐producing CD4⁺ T cells. The results showed that the IFN‐γ and IL‐5 produced, originated from clonally expanded Th1 and Th2, but not Th0 cells, and that the effector functions of long‐term memory cells were unaffected. Adoptive transfer experiments suggested that PT mediated these effects via activation of APC, not by acting on the T cells directly. The effects of PT on the developing T cell response required the presence of the holotoxin (A‐ and B‐subunit); the individual subunits did not show adjuvant effects. The data suggest that PT enhanced cytokine production by promoting differentiation and vigorous clonal expansion of Th1 and Th2 cells via activation of APC.