The human respiratory corona-virus OC43, grown in WI38 cells matures in cytoplasmic vesicles, and its development resembles that of coronavirus 229E. However, we did not observe budding of OC43 virus into the vesicles, although this is a characteristic feature of the morphogenesis of 229E virus. In OC43-infected cells, progeny virus accumulate on the outside surface of the cell in sufficient quantity to bind rat red blood cells and give rise to a “pseudohemadsorption” reaction.