Long term goals of the investigators are to study the pathogenesis of fetal malformations resulting from viral infection during pregnancy. The principle object of the proposed study is to determine the importance of the immune status of the fetus to the pathogenesis of a congenital viral disease. By using the particular model we have selected, we intend to; 1) inject fetuses with vaccine virus or nonreplicating (killed) virus in order to determine the earliest time in fetal development that specific serum neutralizing antibodies to bluetongue virus can be detected in fetal lambs, 2) administer immune serum to fetal lambs immunologically incompetent to bluetongue virus at various times after fetal infection to determine the effectiveness of specific antibody in suppressing the cytopathogenic properties of virus in highly susceptible tissue, and, 3) delay the maturation of immunological competence to bluetongue virus by administering antilymphocyte serum to determine if the cytopathogenic properties of the virus can produce additional lesions in the developing organs. Information obtained from these studies should assist in gaining a better understanding of the influence of some immune mechanisms on the pathogenesis of congenital viral disease.