Interactions of malignant or non-malignant human and rodent cells with the vascular wall were studied using perfused human umbilical cord veins. The integrity of perfused endothelium was confirmed by morphological and functional criteria. Highly malignant cells in vivo adhered to the endothelial cells, as shown by scanning electron microscopy. The specific attachment of radiolabelled malignant cells to the whole vein was already maximal within 30-60 min and remained stable for perfusion flow rates ranging between 10 and 60 ml/min. It increased proportionally to the number of cells infused and could be modulated by human platelets, human fibronectin and rabbit anti-laminin antibodies. In contrast, the binding of human or rodent non-malignant cells in vivo, of human red blood cells and of human platelets to the endothelial cells was negligible under similar experimental conditions. This perfusion system therefore represents a new model for elucidating some mechanisms involved in tumour cell arrest in vivo.