The development of V and S antigens in horse kidney cells infected with equine rhinopneumonitis virus was studied. During the latent period, S antigen was first detected and developed exclusively in the nucleus, whereas V antigen appeared in the cytoplasm with increase in intracellular infectivity. As the virus multiplication proceeded, the whole cell was filled with both antigens, followed by the disappearance of antigenic materials from the cells except the peripheral area at the terminal stage of infection. The development of inclusion materials as revealed by hematoxylin and eosin staining was closely related to the increase of virus titer as well as the development of the viral antigens except at the late stages of infection. These observations together with those by electron microscopy and other methods suggest that in the infected cells S antigen is synthesized in the nucleus, followed by the synthesis of V antigen in the cytoplasm and finally the mature virus particle is completed in the cytoplasm by assembly of both antigenic materials, and that S-antigen represents DNA-protein constituting the viral core and V antigen constitutes the virus particle surface of protein nature.