A model for cell detachment which may influence metastasis in vivo is described involving the transfer of hamster melanoma cells from aggregates of those tumor cells to hamster fibroblast aggregates. The aggregates were made by the spontaneous association of cultured cells. Tumor and fibroblast aggregates were then incubated together but separated by a nylon net that allowed only single cells to pass. Melanoma cells rapidly separated from the tumor aggregates, crossed the net, and attached to the fibroblast aggregates, but fibroblast cells did not. This model of metastasis reflects the postulated role of cell-to-cell adhesion in metastasis and will allow further study of the role of cell attachments in metastasis.