Interactions between CD40 on APC and CD154 (CD40L) expressed by activated CD4+ T cells are crucially involved in formation and function of germinal centers (GC), but mechanistic insight into these interactions remains limited. Functional studies have mostly been restricted to experimental immunization of young-adult inbred SPF rodents that are often genetically manipulated, while studies in humans disallow in vivo manipulation. Therefore, we asked whether a functional antagonist of CD40 interferes with natural GC formation in adult cynomolgus monkeys (Macaca fascicularis) exposed to the environmental antigens of their conventional housing in captivity. Animals were treated with different doses of a unique chimeric antagonist anti-CD40 mAb (ch5D12) and analyzed 1 week or 7 weeks after last injection. Detailed in situ analysis showed that high-dose anti-CD40 treatment increased the ratio of primary over secondary follicles compared to PBS or low-dose treatment, indicative of impairment of the CG reaction. This impairment was reversible since recovery animals, except those with residual anti-CD40 levels, had normalized ratios. Anti-CD40 treatment was associated with decreased antibody production and increased numbers of apoptotic cells in GC. These data demonstrate that CD40-CD154 interactions are pivotal in physiological GC formation in primates responding to environmental antigens, and they support immunotherapeutic strategies using antagonist anti-CD40.