Mammary ducts, and the highly mitotic terminal end buds from which they are derived, consist of two layers of ectodermally derived epithelium, forming a tube-within-a-tube structure. We investigated the role of Ca(2+)-dependent cell-cell adhesion molecules in maintaining the integrity of these layers. Immunostaining showed abundant E-cadherin on the lateral membranes of end bud body cells and ductal lumenal cells, but no P-cadherin. The basally located cap cells and their differentiated descendants, the ductal myoepithelial cells, displayed only P-cadherin. We investigated the functional significance of this pattern of cadherin expression in situ by surgically implanting small, slow-release plastic implants releasing function-blocking antibodies. End buds exposed to a monoclonal antibody to E-cadherin showed disruption of the body epithelium, with epithelial cells floating freely in the lumen. Epithelial DNA synthesis, which is normally very high in these growth buds, abruptly declined. That this reduction in growth was not due to cell damage was shown by spontaneous reaggregation of the cells into a normal epithelium, with resumption of DNA synthesis, when the blocking antibody was depleted. A monoclonal antibody to P-cadherin had no effect on the lumenal layer but partially disrupted the basally located cap cell layer. These data indicate that spatially selective expression of E- and P-cadherins is required for mammary tissue integrity, which is in turn a prerequisite for normal rates of DNA synthesis.