It remains a question whether hematogeneous metastasis arises from a single cancer cell attached to the local endothelium or from a cluster of cancer cells trapped in the vascular bed in the target organ. Adhesive interaction of the single cell form and the clustered form of cancer cells was examined under flow conditions, using two subclones of mouse colon adenocarcinoma Colon 26. A subclone NL17, but not NL14, formed many clusters composed of tumor cells and platelets just after the addition of platelet rich plasma (PRP). Under the shear of 1.0 dyn/cm3, the clustered form of NL17 tethered on laminin or mouse endothelial cell line in hepatic sinusoids (HSE) more frequently than the single cell form of NL17 and NL14. However, all of the clusters showed only transient attachment and never underwent stable arrest on coated laminin, while the single cell form of NL14 and NL17 underwent immediate arrest under shear conditions. On HSE stimulated with TNF-alpha, a small number of NL17 clusters made stable adhesion, although all the clusters detached if the shear stress was increased above 4.0 dyn/cm2. In contrast, the single form of arrested NL17 as well as NL14 remained adherent even at shear of 8.0 dyn/cm2. Compared with single cell, binding of cancer cell clusters to laminin and HSE showed lower resistance to shear stress, although they had adhesive interactions more frequently in flow condition. Since NL17 cells form significantly more metastases by intravenous injection in vivo, our data suggest that "stable adhesion" observed in our flow assay system is not always a prerequisite for clustered cancer cells to develop into metastatic lesions.