A prospective study was conducted to assess the dynamics of the infection and host response to Anaplasma marginale in one closed herd in the dry tropical forest of Costa Rica. The study subjects were the dams and their calves born during 1 breeding season (1995-1996). All cows were sampled at 3 month intervals for antibody detection using a competitive ELISA (cELISA) and for antigen detection using PCR/nonradioactive probe assay. All 24 calves born during the study were individually identified at birth and subsequently sampled each month for PCR and cELISA. Ticks were identified from all animals throughout the entire study period. The results from this study confirmed that the cELISA is a reliable assay for identifying new and carrier infections and that carrier infections can exist at levels below that detectable by PCR. In addition, it was demonstrated that calves born in this region will most likely be exposed to Anaplasma within the first 6 months of age.