Modern vaccines must be designed to generate long-lasting, high-affinity, and broadly neutralizing Ab responses against pathogens. The diversity of B cell clones recruited into germinal center (GC) responses is likely to be important for the Ag-neutralization potential of the Ab-secreting cells and memory cells generated upon immunization. However, the factors that influence the diversity of B cell clones recruited into GCs are unclear. As recirculating naive Ag-specific B cells arrive in Ag-draining secondary lymphoid organs, they may join the ongoing GC response. However, the factors that limit their entry are not well understood, and it is not known how that depends on the stage of the ongoing follicular T cell and GC B cell response. In this article, we show that, in mice, naive B cells have a limited window of time during which they can undergo Ag-driven activation and join ongoing immunization-induced GC responses. However, preloading naive B cells with even a threshold-activating amount of Ag is sufficient to rescue their entry into the GC response during its initiation, peak, and contraction. Based on these results, we suggest that productive acquisition of Ag may be one of the main factors limiting entry of new B cell clones into ongoing immunization-triggered GC responses.