The antibody titers to infectious bursal disease virus (IBDV) of a group of hens were determined every 2 weeks during the laying period using a kinetic-based enzyme-linked immunosorbent assay (ELISA). When the titers of the flock were regressed against time, the flock titer decayed with statistically significant linearity. However, when the antibody titers of individual hens were measured, their titers regressed on time in a significant quartic curvilinear fashion. Since these hens were not reimmunized, this suggests that a anamnestic response was stimulated from an unknown external source.