The human amnion membrane consisting of a single layer of epithelium attached to the continuous basement membrane and the underlying collagenous stroma has been used to study tumor invasion in vitro by means of electron microscopy. Three cell lines utilized in these experiments were established from surgical specimens of human squamous cell carcinomas of the tongue and larynx. The cultivation of amnion membrane for 48h or longer leads to some alterations in the morphological appearance. All three carcinomas form aggregates which appear firmly attached to the amnion epithelium and are able to establish contacts to the basement membrane within 96h. In two cases the basement membrane reveals local defects and some cells have extended processes into the collagenous tissue. However, none of the tumor cells of the three carcinomas have broken through the basement membrane and grown into the collagenous stroma. Some differences of interaction between amnion epithelial cells and tumor cells of the three carcinomas are apparent. These ultrastructural findings show, that the amnion membrane model allows morphological analysis of various steps of tumor invasion under standardized conditions.