Although AMCase activity is not required for protective immunity during acute infection it is required for protection during the chronic stage of infection. Our observation of a higher cyst count and parasite burden as well as decreased survival in AMCase-null mice points to a specific and important role for chitinase mediated cyst lysis in the brain. Thus, within the brain, cyst containment seems as important as the killing of free parasites in the control of pathology. In addition, continuous chitinmediated attack by macrophages and the release of parasites from latent cysts will provide a constant source of antigenic stimulation for the immune response. This latter discovery may provide an explanation for the continuous recruitment of T cells into the brain.