The objective of this study is to determine the mechanisms by which viral infections produce reproductive failure, congenital malformations, and retarded postnatal growth and development. Investigation of the effects of virus infections on mammalian development requires model systems in which cells or organs will differentiate in a controlled environment. The discipline of experimental embryology has produced several such systems. For example, fertilized ova under appropriate conditions will undergo cleavage and blastocyst formation in vitro; embryonic organs such as pancreas can be induced in the presence of another tissue such as salivary gland to undergo morphogenesis in vitro; and cells dispersed by trypsinization from a developing organ such as liver or cartilage will reaggregate into foci having the same histologic pattern as the original organ. The general approach of this study is to introduce viruses into these embryologic models and observe them for the occurrence of developmental abberations. Present efforts are concentrated on the effects of various cytocidal and non-cytocidal viruses on the development of fertilized mouse ova and embryonic tissue.