The membrane mucin Muc4 has been shown to alter cellular behavior through both anti-adhesive effects on cell-cell and cell-extracellular matrix interactions and its ability to act as an intramembrane ligand for the receptor tyrosine kinase ErbB2. The extracellular-regulated kinase (Erk) pathway is regulated by both cell-matrix and cell-cell adhesion. An analysis of the effects of Muc4 expression on Erk phosphorylation in mammary tumor and epithelial cells which exhibit both adhesion dependent growth and contact inhibition of growth showed that the effects are density dependent, with opposing effects on proliferating cells and on contact-inhibited cells. In these cells, cell-matrix interactions through integrins are required activation of the Erk mitogenesis pathway. However, cell-cell interactions via cadherins inhibit the Erk pathway. Expression of Muc4 reverses both of these effects. In contact-inhibited cells, Muc4 appears to activate the Erk pathway at the level of Raf-1; this activation does not depend on Ras activation. The increase in Erk activity correlates with an increase in cyclin D1 expression in these cells. This abrogation of contact inhibition is dependent on the number of mucin repeats in the mucin subunit of Muc4, indicative of an anti-adhesive effect. The mechanism by which Muc4 disrupts contact inhibition involves a Muc4-induced relocalization of E-cadherin from adherens junctions at the lateral membrane of the cells to the apical membrane. Muc4-induced abrogation of contact inhibition may be an important mechanism by which tumors progress from an early more benign state to invasiveness.