Virulent or avirulent strains of type A influenza virus were inoculated into the allantoic cavities of chicken embryos. The antigens of virulent strains appeared initially in the surface epithelium of the allantoic membrane, then in vascular endothelial cells of the chorioallantoic membrane and visceral organs of the embryos, and then spread to parenchymal cells of many organs. In contrast, the antigens of the avirulent strain were confined to the allantoic membrane. These observations indicate that the primary target of virulent influenza viruses in chicken embryos is vascular endothelial cells, and that the embryos died after systemic viral infection.