Pretreatment of embryonal chick heart fragments with ET-18-OCH3 is known to induce resistance to invasion by several malignant cell lines. Embryonal chick heart fragments or cell suspensions prepared from such fragments were explanted on solid substrate and treated in medium with 10 micrograms/ml ET-18-OCH3 or with drug-free medium (control) for 48 h. This medium was washed away and replaced by drug-free fresh medium. Twenty-four to 48 h later the fast plasma membrane movements (involved in ruffling, blebbing, fast shape change and fast translocation) were quantified using a simple method based on subtracting two video images taken with an interval of 28 s. The ET-18-OCH3-treated cells showed a higher intensity of fast plasma membrane movements than control cells. Cells around a treated explant did not show the same radial alignment as in controls, suggesting loss of contact inhibition of movement. Cells from a cell suspension derived from a treated fragment showed faster translocation on solid substrate and faster shape change. We speculate that increased motility of host cells may be involved in resistance to invasion.