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Human amnion cells cultivated in monolayer cultures can easily be infected by poliovirus I, whereas cells of the intact membrane and freshly trypsinized amnion cells are for the most part resistant to infection. Silver impregnation of the histologically intact membrane revealed the presence of a well demarcated intercellular matrix. As the trypsinized amnion cells flatten and spread out on the glass surface, this intercellular substance diminishes considerably, the cells demonstrate marked phagocytic activity and they become susceptible to polioviral infection. It is suggested that this matrix prevents penetration of the virus within the cell either by restricting viral adsorption or by restricting phagocytic activity.