We previously presented a two-dimensional cell motility assay using L-10, a highly metastatic variant of the human rectal adenocarcinoma cell line RCM-1, as a motility model of tumor cells of epithelial origin. In this model, L-10 cells moved outward from the cell islands mainly as a localized coherent sheet of cells when stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA). Electronmicroscopic study of the migrating cell sheets revealed that wide intercellular gaps had developed at the lower portion of the cells, allowing them to extend leading lamellae, whereas close cell-cell contacts remained at the upper portion of the cells. In the present study, the mechanism involved in this localized modulation of cell-cell adhesion at the lower portion of the cells was investigated with special reference to E-cadherin expression. E-cadherin immunostaining, which was demonstrated using an anti-E-cadherin mAb, HECD-1, was decreased in migrating L-10 cell sheets. Apparently, however, E-cadherin was involved in the sheet formation of migrating cells because simultaneous or sequential treatment with TPA and HECD-1 inhibited sheet formation and caused scattering of migrating cells. With immunoelectron microscopic study, E-cadherin immunoreactivity was confined to the upper portion of migrating cells and lost at the lower portion. The level of E-cadherin and alpha-catenin expression was not altered by TPA treatment, although tyrosine phosphorylation of E-cadherin and catenins increased 1.6- to 1.9-fold. We propose that cells are released from cell-cell adhesion only at the lower portion of the cells via phosphorylation of the E-cadherin-catenin complex when stimulated with TPA. This change allows the cells to extend leading lamella and thus move together as coherent sheets (cohort migration).