Recently, fibrinoid of the human placenta has been described as being composed of two main types differing in origin and chemical composition. Fibrin-type fibrinoid is mostly a blood clot product. Matrix-type fibrinoid was defined as the extracellular matrix secreted by extravillous trophoblast cells. The structure and composition of matrix-type fibrinoid was addressed in this study, focusing on fibronectins as one major constituent. A panel of antibodies directed against different fibronectin isoforms generated by different mRNA splicing, as well as antibodies recognizing oncofetal carbohydrate epitopes, were used on cryostat, paraffin and Lowicryl sections of placental tissue from different stages of pregnancy. The oncofetal carbohydrate epitopes studied comprised the blood group precursor antigens i and I. We identified the blood group-related antigen i as an additional marker for matrix-type fibrinoid. The antigen was detected on a glycoprotein that was also recognized by the fibronectin antibodies in western blots. Immunohistochemically this i-glycosylated oncofetal fibronectin-like molecule of about 55 kDa is expressed only by the invasive phenotype of extravillous trophoblast. Long chain carbohydrate moieties with a structure fulfilling the criteria for i reactivity on human placental fibronectin are known to have antiadhesive properties and to enhance resistance of the protein chain to proteolysis. These properties underline the functional relevance of glycosylation of fibronectins in matrix-type fibrinoid and suggest matrix-type fibrinoid is a typical matrix of invasive cells. In contrast, the more mature blood group precursor I could be detected after sialidase pretreatment of sections. This antigen was expressed by villous, non-invasive trophoblast.