To study adhesion, which is probably the initial step in malignant invasion, we associated tissue culture fragments with living substrates in vitro. Malignant HeLa, hepatoma, and PY cells, as well as nonmalignant BHK cells, were transplanted into cultured chick blastoderms and organ fragments from chick embryos. Adhesion was evaluated by time-lapse cinematography, by flushing with Tyrode's solution, and by histological examination after fixation. It was shown that the adhesion of these tissue culture fragments depends on the nature of the substrate. Substrates of connective tissue, mesenchyme, and the basal side of epithelia proved to be adhesive. In contrast, the apical side of intact epithelia was nonadhesive. Perforated epithelia allowed adhesion at the site of the perforation. In the presence of dilysine, HeLa cells adhere to the apical side of epithelia and to the dorsal side of the upper layer of the blastoderm. We concluded that the apical side of intact epithelia constitutes an inappropriate substrate for adhesion of a large variety of cells, in vitro as well as in vivo. Alteration of this characteristic in the presence of dilysine indicates that long-range electrostatic repulsion might be responsible for the nonadhesive character of the epithelia.