Rat virus is a common parvovirus of rats that can cause severe or lethal infection or distort biological responses that require the synthesis of DNA. We have recently shown that inoculation of neonatal or immunodeficient rats leads to persistent infection, whereas inoculation of immunocompetent juvenile rats appears to be shortlived. We also have shown that passively acquired immunity can block persistent-infection. These findings imply that early events determine whether infection is persistent or short-lived and imply that immune,functions may prevent or modulate persistent infection. Aims for continued support stress identification of factors that determine the course and expression of infection and begin to examine strategies for controlling and preventing persistent infection. Experiments will 1) identify early events that influence the course of infection by comparative pathogenesis studies in euthymic and athymic rats, 2) continue characterization of persistent infection using an athymic rat model, 3) determine if infection can be prevented by immunization and 4) determine if host genotype or virus strain influence the expression of infection. These studies will require a variety of techniques including: virus re-isolation and quantification., nucleic acid hybridization in vitro and in situ, serology and immunohistochemistry.