Coxiella burnetii antigen was demonstrated immunocytochemically in the placental tissues of aborted fetuses from four cows and two ewes and from one full-term calf. Antigen was observed in cytoplasmic vacuoles of trophoblasts, especially along the base of the chorionic villi. Positive trophoblasts were of normal size or exhibited striking cytoplasmic enlargement. Antigen was also demonstrated in neutrophils, degenerated cells and extracellularly in the intervillous spaces. The chorioallantoic connective tissue, allantoic surface and fetal tissues were devoid of C. burnetii antigen. Ovine placentas showed strong coxiella-specific staining and histological lesions characterized by moderate to severe, multifocal, necrotizing and purulent inflammation. In bovine placentas, the number of immunopositive cells varied from few to moderate and changes were mild or absent. Placental vasculitis consisting of infiltrating mononuclear cells, neutrophils or eosinophils was observed in both of the ovine and two of the bovine fetuses. In addition, a few intravascular thrombi were observed in one ovine and one bovine placenta. Thus the ovine coxiellosis cases were associated with an abundant accumulation of C. burnetii antigen and with severe necrotizing purulent placentitis, whereas in bovine coxiellosis the lesions were mild or absent and the number of organisms demonstrated immunocytochemically was much smaller. These results suggest a relation between species, lesion severity and weight of infection.