The aggregating properties of murine melanoma cell lines with low metastatic potential (B16-F1) and high metastatic potential (B16-F10 and B16-BL6) were compared. All three types of cells were found to possess Ca²⁺-dependent and Ca²⁺-independent intrinsic mechanisms for cell adhesion, though the extent of reaggregation varied in each mechanism. After trypsin treatment at around 1μg/ml, F10 and BL6 cells reaggregated in the presence of 1mM Ca²⁺ to a greater degree than F1 cells. F10 and BL6 cells were also more aggregative than F1 cells after dissociation with collagenase. The apparent adhesiveness of the cells was found to be dependent on both the manner of cell preparation for reaggregation and on the presence of external Ca²⁺ or serum factors. The results are discussed in relation to the mechanisms of tumor cell arrest with emphasis on the effect of extracellular factors on cell adhesiveness.