The avidity (functional affinity) of specific antibodies are being used to estimate duration of bovine Neospora caninum infection. Here, we report for the first time the avidity pattern in cattle orally inoculated with N. caninum oocysts. In all, 16 pregnant cows and 7 calves were administered N. caninum oocysts. In the cows, the avidity increased during the early course of infection. In all but one, the avidity was < or = 35 during the first 6 weeks after infection and no cow had an avidity value >50 until week 9. The calves were sampled either week 6 (n = 3) or week 9 (n = 9) after infection, and by then had avidities between 2 and 17. The results are in agreement with results from previous investigations of naturally infected cattle, and calves that were experimentally infected with tachyzoites. They further validate the ability of the N. caninum iscom avidity ELISA to accurately assess the duration of infection.