Immunofluorescent studies were performed on placental bed biopsies from 27 normal pregnancies. Uteroplacental arteries were present in 26 of the 27 biopsies and all showed deposition of fibrin in relation to the fibrinoid layer within the vessel wall; 12 also showed deposition of C3. Interstitial fibrinoid deposits also stained positively with fibrin. There was no deposition of immunoglobulins within the arterial changes nor in association with intramural or interstitial trophoblast. This study indicates that fibrin is a significant contributor to the fibrinoid layer within the Uteroplacental arteries, and suggests that the presence of bound C3 is due to the activation of the complement system by fibrinogenesis. There was no evidence that the humoral immune response was involved in the aetiology of the physiological changes in the Uteroplacental arteries.