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SYNOPSIS Histochemical and electron‐microscopical techniques have been employed to study fibrinoid material present in the mouse placenta. These show the material to be an amorphous substance of moderate electron‐density, which stains characteristically with the periodic acid‐Schiff and Hale colloidal iron reactions. Auxiliary techniques, including enzymic digestion, have enabled greater specificity to be obtained from these reactions, and show that the placental fibrinoid contains a mucoprotein rich in tryptophane, with at least two non‐sulphated acid mucopolysaccharides, probably hyaluronic and sialic acids. The fibrinoid is present around each trophoblast cell and as a layer between the foetal and maternal components of the placenta. Considerably more fibrinoid is evident in placentae from hybrid than from inbred matings. It is suggested that the fibrinoid material in the mouse placenta acts as a barrier to prevent the egress of foetal transplantation antigens into the mother.

The placental barrier of the mouse was examined by light and electron microscopy and found to be of the henio-chorial and not of the hemo-endothelial type, as had been previously suggested. The suspected immunological function of the trophoblast is briefly reviewed, and attention drawn to the recent reclassification of the shrew placenta.

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