The contribution of the costimulatory molecules B7–1 and B7–2 to the in vivo differentiation of Th cells remains controversial. The infection of resistant and susceptible strains of mice with the parasite Leishmania major provides a well‐established model for studying in vivo differentiation of CD4+ T cells. We have infected B7–1/B7‐2‐deficient mice onthe BALB/c and 129 background with L. major and subsequently examined different parameters of infection and cytokine responses upon restimulation of lymph node cells in vitro. BALB/c B7–2‐deficient and B7–1/B7–2‐double deficient mice are resistant to L. major, whereas BALB/c B7–1‐deficient mice remain as susceptible as wild‐type BALB/c mice. Differential expression of B7–1 and B7–2 can explain the distinct roles observed for these B7 costimulators in L. major infection.